Dispersion compensation in volume bragg grating-based waveguide display

ABSTRACT

A waveguide display includes a substrate transparent to visible light, a coupler configured to couple display light into the substrate as guided wave in the substrate, and a first VBG and a second VBG coupled to the substrate. The coupler includes a diffractive coupler, a refractive coupler, or a reflective coupler. The first VBG is configured to diffract, at a first region of the first VBG, the display light in the substrate to a first direction, and diffract, at two or more regions of the first VBG along the first direction, the display light from the first region to a second direction towards the second VBG. The second VBG is configured to couple the display light from each of the two or more regions of the first VBG out of the substrate at two or more regions of the second VBG along the second direction.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims priority to U.S. Provisional PatentApplication Ser. No. 62/891,167, filed Aug. 23, 2019, entitled “VolumeBragg Grating-Based Waveguide Display,” the disclosure of which ishereby incorporated by reference in its entirety for all purposes.

BACKGROUND

An artificial reality system, such as a head-mounted display (HMD) orheads-up display (HUD) system, generally includes a near-eye display(e.g., in the form of a headset or a pair of glasses) configured topresent content to a user via an electronic or optic display within, forexample, about 10-20 mm in front of the user's eyes. The near-eyedisplay may display virtual objects or combine images of real objectswith virtual objects, as in virtual reality (VR), augmented reality(AR), or mixed reality (MR) applications. For example, in an AR system,a user may view both images of virtual objects (e.g., computer-generatedimages (CGIs)) and the surrounding environment by, for example, seeingthrough transparent display glasses or lenses (often referred to asoptical see-through).

One example of an optical see-through AR system may use awaveguide-based optical display, where light of projected images may becoupled into a waveguide (e.g., a transparent substrate), propagatewithin the waveguide, and be coupled out of the waveguide at differentlocations. In some implementations, the light of the projected imagesmay be coupled into or out of the waveguide using a diffractive opticalelement, such as a grating. Light from the surrounding environment maypass through a see-through region of the waveguide and reach the user'seyes as well.

SUMMARY

This disclosure relates generally to volume Bragg grating-basedwaveguide displays for near-eye display. More specifically, disclosedherein are techniques for expanding the eyebox, reducing display haze,reducing physical size, improving optical efficiency, reducing opticalartifacts, and increasing field of view of optical see-through near-eyedisplay systems using volume Bragg grating (VBG) couplers. Variousinventive embodiments are described herein, including devices, systems,methods, and the like.

According to some embodiments, a waveguide display may include asubstrate transparent to visible light, and a first VBG, a second VBG,and a third VBG coupled to the substrate. The first VBG may beconfigured to couple display light into the substrate as guided wavetowards a first region of the second VBG. The second VBG may beconfigured to diffract, at the first region of the second VBG, thedisplay light from the first VBG to a first direction (e.g., xdirection), and diffract, at two or more regions of the second VBG alongthe first direction, the display light from the first region to a seconddirection (e.g., y direction) towards the third VBG. The third VBG maybe configured to couple the display light from each of the two or moreregions of the second VBG out of the substrate at two or more regions ofthe third VBG along the second direction. The first VBG and the thirdVBG may have a same grating vector in a plane (e.g., x-y plane)perpendicular to a surface normal direction of the substrate, and mayhave a same grating vector or opposite grating vectors in the surfacenormal (e.g., z) direction of the substrate. In some embodiments, thefirst region of the second VBG and the second region of the second VBGmay have a same grating vector in a plane perpendicular to a surfacenormal direction of the substrate, and may have a same grating vectorand/or opposite grating vectors in the surface normal direction of thesubstrate.

In some embodiments of the waveguide display, the first VBG, the secondVBG, and the third VBG may be configured to diffract the display lightfrom a same field of view range and in a same wavelength range. Each ofthe first VBG, the second VBG, and the third VBG includes a reflectiveVBG or a transmissive VBG. In some embodiments, the third VBG mayinclude a transmissive VBG, and the second VBG may overlap with thethird VBG in a see-through region of the waveguide display.

In some embodiments, at least one of the first VBG, the second VBG, orthe third VBG may include a multiplexed VBG. The first VBG may include afirst set of VBGs, the third VBG may include a second set of VBGs, andeach VBG in the first set of VBGs and a corresponding VBG in the secondset of VBGs may have a same grating vector in a plane perpendicular to asurface normal direction of the substrate and have a same grating vectoror opposite grating vectors in the surface normal direction of thesubstrate, and may be configured to diffract the display light from asame field of view range and in a same wavelength range. In someembodiments, at least one of the first VBG, the second VBG, or the thirdVBG may include VBGs in two or more holographic material layers. In someembodiments, each of the second VBG and the third VBG may becharacterized by a respective thickness less than 100 μm, and thewaveguide display may be characterized by an angular resolution lessthan 2 arcminutes.

In some embodiments, the waveguide display may include a polarizationconvertor between two holographic material layers of the two or moreholographic material layers. In some embodiments, the waveguide displaymay include an anti-reflection layer configured to reduce reflection ofambient light into the substrate. In some embodiments, the waveguidedisplay may include an angular-selective transmissive layer configuredto reflect, diffract, or absorb ambient light incident on theangular-selective transmissive layer with an incidence angle greaterthan a threshold value. In some embodiments, the waveguide display mayinclude a light source configured to generate the display light, andprojector optics configure to collimate the display light and direct thedisplay light to the first VBG.

According to certain embodiments, a waveguide display may include asubstrate transparent to visible light, a coupler configured to coupledisplay light into the substrate as guided wave in the substrate, and afirst VBG and a second VBG coupled to the substrate. The first VBG maybe configured to diffract, at a first region of the first VBG, thedisplay light in the substrate to a first direction, and diffract, attwo or more regions of the first VBG along the first direction, thedisplay light from the first region to a second direction towards thesecond VBG. The second VBG may be configured to couple the display lightfrom each of the two or more regions of the first VBG out of thesubstrate at two or more regions of the second VBG along the seconddirection. Each of the first VBG and the second VBG includes atransmissive VBG or a reflective VBG. In some embodiments, the couplermay include a diffractive coupler, a refractive coupler, or a reflectivecoupler.

In some embodiments, the first VBG may be characterized by a thicknessless than 100 μm, and the waveguide display may be characterized by anangular resolution less than 2 arcminutes. In some embodiments, thesecond VBG may include a transmissive VBG, and the first VBG may overlapwith the second VBG in a see-through region of the waveguide display. Insome embodiments, at least one of the first VBG or the second VBG mayinclude VBGs in two or more holographic material layers. In someembodiments, at least one of the first VBG or the second VBG includes amultiplexed VBG.

This summary is neither intended to identify key or essential featuresof the claimed subject matter, nor is it intended to be used inisolation to determine the scope of the claimed subject matter. Thesubject matter should be understood by reference to appropriate portionsof the entire specification of this disclosure, any or all drawings, andeach claim. The foregoing, together with other features and examples,will be described in more detail below in the following specification,claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments are described in detail below with reference tothe following figures.

FIG. 1 is a simplified block diagram of an example of an artificialreality system environment including a near-eye display system accordingto certain embodiments.

FIG. 2 is a perspective view of an example of a near-eye display systemin the form of a head-mounted display (HMD) device for implementing someof the examples disclosed herein.

FIG. 3 is a perspective view of an example of a near-eye display systemin the form of a pair of glasses for implementing some of the examplesdisclosed herein.

FIG. 4 is a simplified diagram illustrating an example of an opticalsystem in a near-eye display system.

FIG. 5 illustrates an example of an optical see-through augmentedreality system including a waveguide display for exit pupil expansionaccording to certain embodiments.

FIG. 6 illustrates an example of an optical see-through augmentedreality system including a waveguide display for exit pupil expansionaccording to certain embodiments.

FIG. 7A illustrates the spectral bandwidth of an example of a reflectivevolume Bragg grating (VBG) and the spectral bandwidth of an example of atransmissive surface-relief grating (SRG). FIG. 7B illustrates theangular bandwidth of an example of a reflective VBG and the angularbandwidth of an example of a transmissive SRG.

FIG. 8A illustrates an example of an optical see-through augmentedreality system including a waveguide display and surface-relief gratingsfor exit pupil expansion according to certain embodiments. FIG. 8Billustrates an example of an eye box including two-dimensionalreplicated exit pupils according to certain embodiments.

FIG. 9A illustrates wave vectors of light diffracted by examples ofsurface-relief gratings for exit pupil expansion in a waveguide displayand exit pupils for multiple colors.

FIG. 9B illustrates the field-of-view clipping by the examples ofsurface-relief gratings for exit pupil expansion in the waveguidedisplay.

FIG. 10A illustrates an example of a volume Bragg grating-basedwaveguide display according to certain embodiments. FIG. 10B illustratesa top view of the example of the volume Bragg grating-based waveguidedisplay shown in FIG. 10A. FIG. 10C illustrates a side view of theexample of the volume Bragg grating-based waveguide display shown inFIG. 10A.

FIG. 11 illustrates light dispersion in an example of a volume Bragggrating-based waveguide display according to certain embodiments.

FIG. 12A illustrates an example of a volume Bragg grating (VBG). FIG.12B illustrates the Bragg condition for the volume Bragg grating shownin FIG. 12A.

FIG. 13A illustrates an example of a reflective volume Bragg grating ina waveguide display according to certain embodiments. FIG. 13Billustrates an example of a reflective VBG in a waveguide display wherelight diffracted by the reflective VBG is not totally reflected andguided in the waveguide. FIG. 13C illustrates an example of atransmissive volume Bragg grating in a waveguide display according tocertain embodiments. FIG. 13D illustrates an example of a transmissiveVBG in a waveguide display where light diffracted by the transmissiveVBG is not totally reflected and guided in the waveguide.

FIG. 14A illustrates the light dispersion by an example of a reflectivevolume Bragg grating in a waveguide display according to certainembodiments. FIG. 14B illustrates the light dispersion by an example ofa transmissive volume Bragg grating in a waveguide display according tocertain embodiments.

FIG. 15A illustrates a front view of an example of a volume Bragggrating-based waveguide display with exit pupil expansion and dispersionreduction according to certain embodiments. FIG. 15B illustrates a sideview of the example of the volume Bragg grating-based waveguide displayshown in FIG. 15A.

FIG. 16A is a front view of an example of a volume Bragg grating-basedwaveguide display with exit pupil expansion and dispersion reductionaccording to certain embodiments.

FIG. 16B is a side view of the example of the volume Bragg grating-basedwaveguide display shown in FIG. 16A.

FIG. 17A illustrates the propagation of light in different colors andfrom different fields of view in a reflective volume Bragg grating-basedwaveguide display according to certain embodiments. FIG. 17B illustratesthe propagation of light in different colors and from different fieldsof view in a transmissive volume Bragg grating-based waveguide displayaccording to certain embodiments.

FIG. 18 illustrates an example of a reflective volume Bragggrating-based waveguide display with exit pupil expansion and dispersionreduction according to certain embodiments.

FIG. 19 illustrates an example of a transmissive volume Bragggrating-based waveguide display with exit pupil expansion andform-factor reduction according to certain embodiments.

FIG. 20 illustrates another example of a transmissive volume Bragggrating-based waveguide display according to certain embodiments.

FIG. 21 illustrates an example of a volume Bragg grating-based waveguidedisplay with exit pupil expansion, dispersion reduction, and form-factorreduction according to certain embodiments.

FIG. 22A illustrates another example of a volume Bragg grating-basedwaveguide display with exit pupil expansion, dispersion reduction,form-factor reduction, and power efficiency improvement according tocertain embodiments. FIG. 22B illustrates examples of replicated exitpupils at an eyebox of the volume Bragg grating-based waveguide displayshown in FIG. 22A.

FIG. 23A illustrates an example of a volume Bragg grating-basedwaveguide display with exit pupil expansion, dispersion reduction, andform-factor reduction according to certain embodiments. FIG. 23Billustrates an example of a volume Bragg grating-based waveguide displaywith exit pupil expansion, dispersion reduction, form-factor reduction,and power efficiency improvement according to certain embodiments.

FIG. 24A is a front view of an example of a volume Bragg grating-basedwaveguide display including an image projector and multiple polymerlayers according to certain embodiments. FIG. 24B is a side view of theexample of the volume Bragg grating-based waveguide display includingthe image projector and multiple polymer layers according to certainembodiments.

FIG. 25 illustrates an example of a volume Bragg grating-based waveguidedisplay including multiple grating layers for different fields of viewand/or light wavelengths according to certain embodiments.

FIG. 26 illustrates an example of a waveguide display including twomultiplexed volume Bragg gratings and a polarization convertor betweenthe two multiplexed volume Bragg gratings according to certainembodiments.

FIG. 27 illustrates an example of a waveguide display including ananti-reflection layer and an angular-selective transmissive layeraccording to certain embodiments.

FIG. 28 is a simplified block diagram of an example electronic system ofan example near-eye display according to certain embodiments.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated may be employed without departing from theprinciples, or benefits touted, of this disclosure.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION

This disclosure relates generally to volume Bragg grating (VBG)-basedwaveguide display for near-eye display systems. In a near-eye displaysystem, it is generally desirable to expand the eyebox, reduce displayhaze, improve image quality (e.g., resolution and contrast), reducephysical size, increase power efficiency, and increase the field ofview. In a waveguide-based near-eye display system, light of projectedimages may be coupled into a waveguide (e.g., a transparent substrate),propagate within the waveguide, and be coupled out of the waveguide atdifferent locations to replicate exit pupils and expand the eyebox. Twoor more gratings may be used to expand the exit pupil in two dimensions.In a waveguide-based near-eye display system for augmented realityapplications, light from the surrounding environment may pass through atleast a see-through region of the waveguide display (e.g., thetransparent substrate) and reach the user's eyes. In someimplementations, the light of the projected images may be coupled intoor out of the waveguide using diffractive optical elements, such asgratings. Couplers implemented using diffractive optical elements maycause dispersion between light of different colors due to the wavelengthdependency of light diffraction. Therefore, images of different colorcomponents in a color image may not overlap and thus the resolution ofthe displayed image may be reduced. To reduce the dispersion and improvethe resolution, thick transmissive and/or reflective VBG gratings may beused, which may be impractical in many cases and/or may causesignificant display haze. For example, in some cases, transmissive VBGgratings with a thickness of greater than 1 mm may be used to achieve adesired resolution performance. Reflective VBG gratings with a lowerthickness may be used to achieve the desired resolution performance.However, with reflection gratings, the gratings for two-dimensionalpupil expansion may not overlap and thus the physical size of thewaveguide display may be large.

According to certain embodiments, two VBG gratings (or two portions of asame grating) with matching grating vectors (e.g., having the samegrating vector in a plane perpendicular to a surface normal direction ofthe transparent substrate and having the same and/or opposite gratingvectors in the surface-normal direction of the transparent substrate,but recorded in different exposure durations to achieve differentdiffraction efficiencies) may be used to diffract display light andexpand the exit pupil in one dimension. The two VBG gratings maycompensate for the dispersion of display light caused by each other toreduce the overall dispersion, due to the opposite Bragg conditions(e.g., +1 order and −1 order diffractions) at the two VBG gratings.Therefore, thin VBG gratings may be used to achieve the desiredresolution. Because of the dispersion compensation, thin transmissiveVBG gratings may be used to achieve the desired resolution, and thegratings for the two-dimensional pupil expansion may at least partiallyoverlap to reduce the physical size of the waveguide display.

In some embodiments, a first pair of VBG gratings (or two portions of agrating) may be used to expand the exit pupil in one dimension andcompensate for the dispersion caused by each other, and a second pair ofVBG gratings (or two portions of a grating) may be used to expand theexit pupil in another dimension and may compensate for the dispersioncaused by each other. Thus, the exit pupil may be replicated in twodimensions and the resolution of the displayed images may be high inboth dimensions.

In the following description, various inventive embodiments aredescribed, including devices, systems, methods, and the like. For thepurposes of explanation, specific details are set forth in order toprovide a thorough understanding of examples of the disclosure. However,it will be apparent that various examples may be practiced without thesespecific details. For example, devices, systems, structures, assemblies,methods, and other components may be shown as components in blockdiagram form in order not to obscure the examples in unnecessary detail.In other instances, well-known devices, processes, systems, structures,and techniques may be shown without necessary detail in order to avoidobscuring the examples. The figures and description are not intended tobe restrictive. The terms and expressions that have been employed inthis disclosure are used as terms of description and not of limitation,and there is no intention in the use of such terms and expressions ofexcluding any equivalents of the features shown and described orportions thereof. The word “example” is used herein to mean “serving asan example, instance, or illustration.” Any embodiment or designdescribed herein as “example” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs.

FIG. 1 is a simplified block diagram of an example of an artificialreality system environment 100 including a near-eye display 120 inaccordance with certain embodiments. Artificial reality systemenvironment 100 shown in FIG. 1 may include near-eye display 120, anoptional external imaging device 150, and an optional input/outputinterface 140, each of which may be coupled to an optional console 110.While FIG. 1 shows an example of artificial reality system environment100 including one near-eye display 120, one external imaging device 150,and one input/output interface 140, any number of these components maybe included in artificial reality system environment 100, or any of thecomponents may be omitted. For example, there may be multiple near-eyedisplays 120 monitored by one or more external imaging devices 150 incommunication with console 110. In some configurations, artificialreality system environment 100 may not include external imaging device150, optional input/output interface 140, and optional console 110. Inalternative configurations, different or additional components may beincluded in artificial reality system environment 100.

Near-eye display 120 may be a head-mounted display that presents contentto a user. Examples of content presented by near-eye display 120 includeone or more of images, videos, audio, or any combination thereof. Insome embodiments, audio may be presented via an external device (e.g.,speakers and/or headphones) that receives audio information fromnear-eye display 120, console 110, or both, and presents audio databased on the audio information. Near-eye display 120 may include one ormore rigid bodies, which may be rigidly or non-rigidly coupled to eachother. A rigid coupling between rigid bodies may cause the coupled rigidbodies to act as a single rigid entity. A non-rigid coupling betweenrigid bodies may allow the rigid bodies to move relative to each other.In various embodiments, near-eye display 120 may be implemented in anysuitable form-factor, including a pair of glasses. Some embodiments ofnear-eye display 120 are further described below with respect to FIGS. 2and 3. Additionally, in various embodiments, the functionality describedherein may be used in a headset that combines images of an environmentexternal to near-eye display 120 and artificial reality content (e.g.,computer-generated images). Therefore, near-eye display 120 may augmentimages of a physical, real-world environment external to near-eyedisplay 120 with generated content (e.g., images, video, sound, etc.) topresent an augmented reality to a user.

In various embodiments, near-eye display 120 may include one or more ofdisplay electronics 122, display optics 124, and an eye-tracking unit130. In some embodiments, near-eye display 120 may also include one ormore locators 126, one or more position sensors 128, and an inertialmeasurement unit (IMU) 132. Near-eye display 120 may omit any ofeye-tracking unit 130, locators 126, position sensors 128, and IMU 132,or include additional elements in various embodiments. Additionally, insome embodiments, near-eye display 120 may include elements combiningthe function of various elements described in conjunction with FIG. 1.

Display electronics 122 may display or facilitate the display of imagesto the user according to data received from, for example, console 110.In various embodiments, display electronics 122 may include one or moredisplay panels, such as a liquid crystal display (LCD), an organic lightemitting diode (OLED) display, an inorganic light emitting diode (ILED)display, a micro light emitting diode (μLED) display, an active-matrixOLED display (AMOLED), a transparent OLED display (TOLED), or some otherdisplay. For example, in one implementation of near-eye display 120,display electronics 122 may include a front TOLED panel, a rear displaypanel, and an optical component (e.g., an attenuator, polarizer, ordiffractive or spectral film) between the front and rear display panels.Display electronics 122 may include pixels to emit light of apredominant color such as red, green, blue, white, or yellow. In someimplementations, display electronics 122 may display a three-dimensional(3D) image through stereoscopic effects produced by two-dimensionalpanels to create a subjective perception of image depth. For example,display electronics 122 may include a left display and a right displaypositioned in front of a user's left eye and right eye, respectively.The left and right displays may present copies of an image shiftedhorizontally relative to each other to create a stereoscopic effect(e.g., a perception of image depth by a user viewing the image).

In certain embodiments, display optics 124 may display image contentoptically (e.g., using optical waveguides and couplers) or magnify imagelight received from display electronics 122, correct optical errorsassociated with the image light, and present the corrected image lightto a user of near-eye display 120. In various embodiments, displayoptics 124 may include one or more optical elements, such as, forexample, a substrate, optical waveguides, an aperture, a Fresnel lens, aconvex lens, a concave lens, a filter, input/output couplers, or anyother suitable optical elements that may affect image light emitted fromdisplay electronics 122. Display optics 124 may include a combination ofdifferent optical elements as well as mechanical couplings to maintainrelative spacing and orientation of the optical elements in thecombination. One or more optical elements in display optics 124 may havean optical coating, such as an anti-reflective coating, a reflectivecoating, a filtering coating, or a combination of different opticalcoatings.

Magnification of the image light by display optics 124 may allow displayelectronics 122 to be physically smaller, weigh less, and consume lesspower than larger displays. Additionally, magnification may increase afield of view of the displayed content. The amount of magnification ofimage light by display optics 124 may be changed by adjusting, adding,or removing optical elements from display optics 124. In someembodiments, display optics 124 may project displayed images to one ormore image planes that may be further away from the user's eyes thannear-eye display 120.

Display optics 124 may also be designed to correct one or more types ofoptical errors, such as two-dimensional optical errors,three-dimensional optical errors, or any combination thereof.Two-dimensional errors may include optical aberrations that occur in twodimensions. Example types of two-dimensional errors may include barreldistortion, pincushion distortion, longitudinal chromatic aberration,and transverse chromatic aberration. Three-dimensional errors mayinclude optical errors that occur in three dimensions. Example types ofthree-dimensional errors may include spherical aberration, comaticaberration, field curvature, and astigmatism.

Locators 126 may be objects located in specific positions on near-eyedisplay 120 relative to one another and relative to a reference point onnear-eye display 120. In some implementations, console 110 may identifylocators 126 in images captured by external imaging device 150 todetermine the artificial reality headset's position, orientation, orboth. A locator 126 may be an LED, a corner cube reflector, a reflectivemarker, a type of light source that contrasts with an environment inwhich near-eye display 120 operates, or any combination thereof. Inembodiments where locators 126 are active components (e.g., LEDs orother types of light emitting devices), locators 126 may emit light inthe visible band (e.g., about 380 nm to 750 nm), in the infrared (IR)band (e.g., about 750 nm to 1 mm), in the ultraviolet band (e.g., about10 nm to about 380 nm), in another portion of the electromagneticspectrum, or in any combination of portions of the electromagneticspectrum.

External imaging device 150 may include one or more cameras, one or morevideo cameras, any other device capable of capturing images includingone or more of locators 126, or any combination thereof. Additionally,external imaging device 150 may include one or more filters (e.g., toincrease signal to noise ratio). External imaging device 150 may beconfigured to detect light emitted or reflected from locators 126 in afield of view of external imaging device 150. In embodiments wherelocators 126 include passive elements (e.g., retroreflectors), externalimaging device 150 may include a light source that illuminates some orall of locators 126, which may retro-reflect the light to the lightsource in external imaging device 150. Slow calibration data may becommunicated from external imaging device 150 to console 110, andexternal imaging device 150 may receive one or more calibrationparameters from console 110 to adjust one or more imaging parameters(e.g., focal length, focus, frame rate, sensor temperature, shutterspeed, aperture, etc.).

Position sensors 128 may generate one or more measurement signals inresponse to motion of near-eye display 120. Examples of position sensors128 may include accelerometers, gyroscopes, magnetometers, othermotion-detecting or error-correcting sensors, or any combinationthereof. For example, in some embodiments, position sensors 128 mayinclude multiple accelerometers to measure translational motion (e.g.,forward/back, up/down, or left/right) and multiple gyroscopes to measurerotational motion (e.g., pitch, yaw, or roll). In some embodiments,various position sensors may be oriented orthogonally to each other.

IMU 132 may be an electronic device that generates fast calibration databased on measurement signals received from one or more of positionsensors 128. Position sensors 128 may be located external to IMU 132,internal to IMU 132, or any combination thereof. Based on the one ormore measurement signals from one or more position sensors 128, IMU 132may generate fast calibration data indicating an estimated position ofnear-eye display 120 relative to an initial position of near-eye display120. For example, IMU 132 may integrate measurement signals receivedfrom accelerometers over time to estimate a velocity vector andintegrate the velocity vector over time to determine an estimatedposition of a reference point on near-eye display 120. Alternatively,IMU 132 may provide the sampled measurement signals to console 110,which may determine the fast calibration data. While the reference pointmay generally be defined as a point in space, in various embodiments,the reference point may also be defined as a point within near-eyedisplay 120 (e.g., a center of IMU 132).

Eye-tracking unit 130 may include one or more eye-tracking systems. Eyetracking may refer to determining an eye's position, includingorientation and location of the eye, relative to near-eye display 120.An eye-tracking system may include an imaging system to image one ormore eyes and may optionally include a light emitter, which may generatelight that is directed to an eye such that light reflected by the eyemay be captured by the imaging system. For example, eye-tracking unit130 may include a non-coherent or coherent light source (e.g., a laserdiode) emitting light in the visible spectrum or infrared spectrum, anda camera capturing the light reflected by the user's eye. As anotherexample, eye-tracking unit 130 may capture reflected radio waves emittedby a miniature radar unit. Eye-tracking unit 130 may use low-power lightemitters that emit light at frequencies and intensities that would notinjure the eye or cause physical discomfort. Eye-tracking unit 130 maybe arranged to increase contrast in images of an eye captured byeye-tracking unit 130 while reducing the overall power consumed byeye-tracking unit 130 (e.g., reducing power consumed by a light emitterand an imaging system included in eye-tracking unit 130). For example,in some implementations, eye-tracking unit 130 may consume less than 100milliwatts of power.

Near-eye display 120 may use the orientation of the eye to, e.g.,determine an inter-pupillary distance (IPD) of the user, determine gazedirection, introduce depth cues (e.g., blur image outside of the user'smain line of sight), collect heuristics on the user interaction in theVR media (e.g., time spent on any particular subject, object, or frameas a function of exposed stimuli), some other functions that are basedin part on the orientation of at least one of the user's eyes, or anycombination thereof. Because the orientation may be determined for botheyes of the user, eye-tracking unit 130 may be able to determine wherethe user is looking. For example, determining a direction of a user'sgaze may include determining a point of convergence based on thedetermined orientations of the user's left and right eyes. A point ofconvergence may be the point where the two foveal axes of the user'seyes intersect. The direction of the user's gaze may be the direction ofa line passing through the point of convergence and the mid-pointbetween the pupils of the user's eyes.

Input/output interface 140 may be a device that allows a user to sendaction requests to console 110. An action request may be a request toperform a particular action. For example, an action request may be tostart or to end an application or to perform a particular action withinthe application. Input/output interface 140 may include one or moreinput devices. Example input devices may include a keyboard, a mouse, agame controller, a glove, a button, a touch screen, or any othersuitable device for receiving action requests and communicating thereceived action requests to console 110. An action request received bythe input/output interface 140 may be communicated to console 110, whichmay perform an action corresponding to the requested action. In someembodiments, input/output interface 140 may provide haptic feedback tothe user in accordance with instructions received from console 110. Forexample, input/output interface 140 may provide haptic feedback when anaction request is received, or when console 110 has performed arequested action and communicates instructions to input/output interface140. In some embodiments, external imaging device 150 may be used totrack input/output interface 140, such as tracking the location orposition of a controller (which may include, for example, an IR lightsource) or a hand of the user to determine the motion of the user. Insome embodiments, near-eye display 120 may include one or more imagingdevices to track input/output interface 140, such as tracking thelocation or position of a controller or a hand of the user to determinethe motion of the user.

Console 110 may provide content to near-eye display 120 for presentationto the user in accordance with information received from one or more ofexternal imaging device 150, near-eye display 120, and input/outputinterface 140. In the example shown in FIG. 1, console 110 may includean application store 112, a headset tracking module 114, an artificialreality engine 116, and an eye-tracking module 118. Some embodiments ofconsole 110 may include different or additional modules than thosedescribed in conjunction with FIG. 1. Functions further described belowmay be distributed among components of console 110 in a different mannerthan is described here.

In some embodiments, console 110 may include a processor and anon-transitory computer-readable storage medium storing instructionsexecutable by the processor. The processor may include multipleprocessing units executing instructions in parallel. The non-transitorycomputer-readable storage medium may be any memory, such as a hard diskdrive, a removable memory, or a solid-state drive (e.g., flash memory ordynamic random access memory (DRAM)). In various embodiments, themodules of console 110 described in conjunction with FIG. 1 may beencoded as instructions in the non-transitory computer-readable storagemedium that, when executed by the processor, cause the processor toperform the functions further described below.

Application store 112 may store one or more applications for executionby console 110. An application may include a group of instructions that,when executed by a processor, generates content for presentation to theuser. Content generated by an application may be in response to inputsreceived from the user via movement of the user's eyes or inputsreceived from the input/output interface 140. Examples of theapplications may include gaming applications, conferencing applications,video playback application, or other suitable applications.

Headset tracking module 114 may track movements of near-eye display 120using slow calibration information from external imaging device 150. Forexample, headset tracking module 114 may determine positions of areference point of near-eye display 120 using observed locators from theslow calibration information and a model of near-eye display 120.Headset tracking module 114 may also determine positions of a referencepoint of near-eye display 120 using position information from the fastcalibration information. Additionally, in some embodiments, headsettracking module 114 may use portions of the fast calibrationinformation, the slow calibration information, or any combinationthereof, to predict a future location of near-eye display 120. Headsettracking module 114 may provide the estimated or predicted futureposition of near-eye display 120 to artificial reality engine 116.

Artificial reality engine 116 may execute applications within artificialreality system environment 100 and receive position information ofnear-eye display 120, acceleration information of near-eye display 120,velocity information of near-eye display 120, predicted future positionsof near-eye display 120, or any combination thereof from headsettracking module 114. Virtual reality engine 116 may also receiveestimated eye position and orientation information from eye-trackingmodule 118. Based on the received information, artificial reality engine116 may determine content to provide to near-eye display 120 forpresentation to the user. For example, if the received informationindicates that the user has looked to the left, artificial realityengine 116 may generate content for near-eye display 120 that mirrorsthe user's eye movement in a virtual environment. Additionally,artificial reality engine 116 may perform an action within anapplication executing on console 110 in response to an action requestreceived from input/output interface 140, and provide feedback to theuser indicating that the action has been performed. The feedback may bevisual or audible feedback via near-eye display 120 or haptic feedbackvia input/output interface 140.

Eye-tracking module 118 may receive eye-tracking data from eye-trackingunit 130 and determine the position of the user's eye based on the eyetracking data. The position of the eye may include an eye's orientation,location, or both relative to near-eye display 120 or any elementthereof. Because the eye's axes of rotation change as a function of theeye's location in its socket, determining the eye's location in itssocket may allow eye-tracking module 118 to more accurately determinethe eye's orientation.

FIG. 2 is a perspective view of an example of a near-eye display in theform of an HMD device 200 for implementing some of the examplesdisclosed herein. HMD device 200 may be a part of, e.g., a VR system, anAR system, an MR system, or any combination thereof. HMD device 200 mayinclude a body 220 and a head strap 230. FIG. 2 shows a top side 223, afront side 225, and a right side 227 of body 220 in the perspectiveview. Head strap 230 may have an adjustable or extendible length. Theremay be a sufficient space between body 220 and head strap 230 of HMDdevice 200 for allowing a user to mount HMD device 200 onto the user'shead. In various embodiments, HMD device 200 may include additional,fewer, or different components. For example, in some embodiments, HMDdevice 200 may include eyeglass temples and temple tips as shown in, forexample, FIG. 3 below, rather than head strap 230.

HMD device 200 may present to a user media including virtual and/oraugmented views of a physical, real-world environment withcomputer-generated elements. Examples of the media presented by HMDdevice 200 may include images (e.g., two-dimensional (2D) orthree-dimensional (3D) images), videos (e.g., 2D or 3D videos), audio,or any combination thereof. The images and videos may be presented toeach eye of the user by one or more display assemblies (not shown inFIG. 2) enclosed in body 220 of HMD device 200. In various embodiments,the one or more display assemblies may include a single electronicdisplay panel or multiple electronic display panels (e.g., one displaypanel for each eye of the user). Examples of the electronic displaypanel(s) may include, for example, an LCD, an OLED display, an ILEDdisplay, a μLED display, an AMOLED, a TOLED, some other display, or anycombination thereof. HMD device 200 may include two eyebox regions.

In some implementations, HMD device 200 may include various sensors (notshown), such as depth sensors, motion sensors, position sensors, and eyetracking sensors. Some of these sensors may use a structured lightpattern for sensing. In some implementations, HMD device 200 may includean input/output interface for communicating with a console. In someimplementations, HMD device 200 may include a virtual reality engine(not shown) that can execute applications within HMD device 200 andreceive depth information, position information, accelerationinformation, velocity information, predicted future positions, or anycombination thereof of HMD device 200 from the various sensors. In someimplementations, the information received by the virtual reality enginemay be used for producing a signal (e.g., display instructions) to theone or more display assemblies. In some implementations, HMD device 200may include locators (not shown, such as locators 126) located in fixedpositions on body 220 relative to one another and relative to areference point. Each of the locators may emit light that is detectableby an external imaging device.

FIG. 3 is a perspective view of an example of a near-eye display 300 inthe form of a pair of glasses for implementing some of the examplesdisclosed herein. Near-eye display 300 may be a specific implementationof near-eye display 120 of FIG. 1, and may be configured to operate as avirtual reality display, an augmented reality display, and/or a mixedreality display. Near-eye display 300 may include a frame 305 and adisplay 310. Display 310 may be configured to present content to a user.In some embodiments, display 310 may include display electronics and/ordisplay optics. For example, as described above with respect to near-eyedisplay 120 of FIG. 1, display 310 may include an LCD display panel, anLED display panel, or an optical display panel (e.g., a waveguidedisplay assembly).

Near-eye display 300 may further include various sensors 350 a, 350 b,350 c, 350 d, and 350 e on or within frame 305. In some embodiments,sensors 350 a-350 e may include one or more depth sensors, motionsensors, position sensors, inertial sensors, or ambient light sensors.In some embodiments, sensors 350 a-350 e may include one or more imagesensors configured to generate image data representing different fieldsof views in different directions. In some embodiments, sensors 350 a-350e may be used as input devices to control or influence the displayedcontent of near-eye display 300, and/or to provide an interactiveVR/AR/MR experience to a user of near-eye display 300. In someembodiments, sensors 350 a-350 e may also be used for stereoscopicimaging.

In some embodiments, near-eye display 300 may further include one ormore illuminators 330 to project light into the physical environment.The projected light may be associated with different frequency bands(e.g., visible light, infra-red light, ultra-violet light, etc.), andmay serve various purposes. For example, illuminator(s) 330 may projectlight in a dark environment (or in an environment with low intensity ofinfra-red light, ultra-violet light, etc.) to assist sensors 350 a-350 ein capturing images of different objects within the dark environment. Insome embodiments, illuminator(s) 330 may be used to project certainlight pattern onto the objects within the environment. In someembodiments, illuminator(s) 330 may be used as locators, such aslocators 126 described above with respect to FIG. 1.

In some embodiments, near-eye display 300 may also include ahigh-resolution camera 340. Camera 340 may capture images of thephysical environment in the field of view. The captured images may beprocessed, for example, by a virtual reality engine (e.g., artificialreality engine 116 of FIG. 1) to add virtual objects to the capturedimages or modify physical objects in the captured images, and theprocessed images may be displayed to the user by display 310 for AR orMR applications.

FIG. 4 is a simplified diagram illustrating an example of an opticalsystem 400 in a near-eye display system. Optical system 400 may includean image source 410 and projector optics 420. In the example shown inFIG. 4, image source 410 is in front of projector optics 420. In variousembodiments, image source 410 may be located outside of the field ofview of user's eye 490. For example, one or more reflectors ordirectional couplers may be used to deflect light from an image sourcethat is outside of the field of view of user's eye 490 to make the imagesource appear to be at the location of image source 410 shown in FIG. 4.Light from an area (e.g., a pixel or a light emitting device) on imagesource 410 may be collimated and directed to an exit pupil 430 byprojector optics 420. Thus, objects at different spatial locations onimage source 410 may appear to be objects far away from user's eye 490in different viewing angles (FOVs). The collimated light from differentviewing angles may then be focused by the lens of user's eye 490 ontodifferent locations on retina 492 of user's eye 490. For example, atleast some portions of the light may be focused on a fovea 494 on retina492. Collimated light rays from an area on image source 410 and incidenton user's eye 490 from a same direction may be focused onto a samelocation on retina 492. As such, a single image of image source 410 maybe formed on retina 492.

The user experience of using an artificial reality system may depend onseveral characteristics of the optical system, including field of view(FOV), image quality (e.g., angular resolution), size of the eyebox (toaccommodate for eye and head movements), and brightness of the light (orcontrast) within the eyebox. Field of view describes the angular rangeof the image as seen by the user, usually measured in degrees asobserved by one eye (for a monocular HMD) or both eyes (for eitherbiocular or binocular HMDs). The human visual system may have a totalbinocular FOV of about 200° (horizontal) by 130° (vertical). To create afully immersive visual environment, a large FOV is desirable because alarge FOV (e.g., greater than about 60°) may provide a sense of “beingin” an image, rather than merely viewing the image. Smaller fields ofview may also preclude some important visual information. For example,an HMD system with a small FOV may use a gesture interface, but theusers may not see their hands in the small FOV to be sure that they areusing the correct motions. On the other hand, wider fields of view mayrequire larger displays or optical systems, which may influence thesize, weight, cost, and comfort of using the HMD.

Resolution may refer to the angular size of a displayed pixel or imageelement appearing to a user, or the ability for the user to view andcorrectly interpret an object as imaged by a pixel and/or other pixels.The resolution of an HMD may be specified as the number of pixels on theimage source for a given FOV value, from which an angular resolution maybe determined by dividing the FOV in one direction by the number ofpixels in the same direction on the image source. For example, for ahorizontal FOV of 40° and 1080 pixels in the horizontal direction on theimage source, the corresponding angular resolution may be about 2.2arc-minutes, compared with the one-arc-minute resolution associated withSnellen 20/20 human visual acuity.

In some cases, the eyebox may be a two-dimensional box in front of theuser's eye, from which the displayed image from the image source may beviewed. If the pupil of the user moves outside of the eyebox, thedisplayed image may not be seen by the user. For example, in anon-pupil-forming configuration, there exists a viewing eyebox withinwhich there will be unvignetted viewing of the HMD image source, and thedisplayed image may vignette or may be clipped but may still be viewablewhen the pupil of user's eye is outside of the viewing eyebox. In apupil-forming configuration, the image may not be viewable outside theexit pupil.

The fovea of a human eye, where the highest resolution may be achievedon the retina, may correspond to an FOV of about 2° to about 3°. Thismay require that the eye rotates in order to view off-axis objects witha highest resolution. The rotation of the eye to view the off-axisobjects may introduce a translation of the pupil because the eye rotatesaround a point that is about 10 mm behind the pupil. In addition, a usermay not always be able to accurately position the pupil (e.g., having aradius of about 2.5 mm) of the user's eye at an ideal location in theeyebox. Furthermore, the environment where the HMD is used may requirethe eyebox to be larger to allow for movement of the user's eye and/orhead relative the HMD, for example, when the HMD is used in a movingvehicle or designed to be used while the user is moving on foot. Theamount of movement in these situations may depend on how well the HMD iscoupled to the user's head.

Thus, the optical system of the HMD may need to provide a sufficientlylarge exit pupil or viewing eyebox for viewing the full FOV with fullresolution, in order to accommodate the movements of the user's pupilrelative to the HMD. For example, in a pupil-forming configuration, aminimum size of 12 mm to 15 mm may be desired for the exit pupil. If theeyebox is too small, minor misalignments between the eye and the HMD mayresult in at least partial loss of the image, and the user experiencemay be substantially impaired. In general, the lateral extent of theeyebox is more critical than the vertical extent of the eyebox. This maybe in part due to the significant variances in eye separation distancebetween users, and the fact that misalignments to eyewear tend to morefrequently occur in the lateral dimension and users tend to morefrequently adjust their gaze left and right, and with greater amplitude,than adjusting the gaze up and down. Thus, techniques that can increasethe lateral dimension of the eyebox may substantially improve a user'sexperience with an HMD. On the other hand, the larger the eyebox, thelarger the optics and the heavier and bulkier the near-eye displaydevice may be.

In order to view the displayed image against a bright background, theimage source of an AR HMD may need to be sufficiently bright, and theoptical system may need to be efficient to provide a bright image to theuser's eye such that the displayed image may be visible in a backgroundincluding strong ambient light, such as sunlight. The optical system ofan HMD may be designed to concentrate light in the eyebox. When theeyebox is large, an image source with high power may be used to providea bright image viewable within the large eyebox. Thus, there may betrade-offs among the size of the eyebox, cost, brightness, opticalcomplexity, image quality, and size and weight of the optical system.

FIG. 5 illustrates an example of an optical see-through augmentedreality system 500 including a waveguide display for exit pupilexpansion according to certain embodiments. Augmented reality system 500may include a projector 510 and a combiner 515. Projector 510 mayinclude a light source or image source 512 and projector optics 514. Insome embodiments, light source or image source 512 may include one ormore micro-LED devices. In some embodiments, image source 512 mayinclude a plurality of pixels that displays virtual objects, such as anLCD display panel or an LED display panel. In some embodiments, imagesource 512 may include a light source that generates coherent orpartially coherent light. For example, image source 512 may include alaser diode, a vertical cavity surface emitting laser, an LED, asuperluminescent LED (sLED), and/or a micro-LED described above. In someembodiments, image source 512 may include a plurality of light sources(e.g., an array of micro-LEDs described above) each emitting amonochromatic image light corresponding to a primary color (e.g., red,green, or blue). In some embodiments, image source 512 may include threetwo-dimensional arrays of micro-LEDs, where each two-dimensional arrayof micro-LEDs may include micro-LEDs configured to emit light of aprimary color (e.g., red, green, or blue). In some embodiments, imagesource 512 may include an optical pattern generator, such as a spatiallight modulator. Projector optics 514 may include one or more opticalcomponents that can condition the light from image source 512, such asexpanding, collimating, scanning, or projecting light from image source512 to combiner 515. The one or more optical components may include, forexample, one or more lenses, liquid lenses, mirrors, free-form optics,apertures, and/or gratings. For example, in some embodiments, imagesource 512 may include one or more one-dimensional arrays or elongatedtwo-dimensional arrays of micro-LEDs, and projector optics 514 mayinclude one or more one-dimensional scanners (e.g., micro-mirrors orprisms) configured to scan the one-dimensional arrays or elongatedtwo-dimensional arrays of micro-LEDs to generate image frames. In someembodiments, projector optics 514 may include a liquid lens (e.g., aliquid crystal lens) with a plurality of electrodes that allows scanningof the light from image source 512.

Combiner 515 may include an input coupler 530 for coupling light fromprojector 510 into a substrate 520 of combiner 515. Input coupler 530may include a volume holographic grating or another diffractive opticalelement (DOE) (e.g., a surface-relief grating (SRG)), a slantedreflective surface of substrate 520, or a refractive coupler (e.g., awedge or a prism). Input coupler 530 may have a coupling efficiency ofgreater than 30%, 50%, 75%, 90%, or higher for visible light. Visiblelight coupled into substrate 520 may propagate within substrate 520through, for example, total internal reflection (TIR). Substrate 520 maybe in the form of a lens of a pair of eyeglasses. Substrate 520 may havea flat or a curved surface, and may include one or more types ofdielectric materials, such as glass, quartz, plastic, polymer,poly(methyl methacrylate) (PMMA), crystal, ceramic, or the like. Athickness of the substrate may range from, for example, less than about1 mm to about 10 mm or more. Substrate 520 may be transparent to visiblelight.

Substrate 520 may include or may be coupled to a plurality of outputcouplers 540 each configured to extract at least a portion of the lightguided by and propagating within substrate 520 from substrate 520, anddirect extracted light 560 to an eyebox 595 where an eye 590 of the userof augmented reality system 500 may be located when augmented realitysystem 500 is in use. The plurality of output couplers 540 may replicatethe exit pupil to increase the size of eyebox 595, such that thedisplayed image may be visible in a larger area. As input coupler 530,output couplers 540 may include grating couplers (e.g., volumeholographic gratings or surface-relief gratings), other diffractionoptical elements (DOEs), prisms, etc. Output couplers 540 may havedifferent coupling (e.g., diffraction) efficiencies at differentlocations. Substrate 520 may also allow light 550 from the environmentin front of combiner 515 to pass through with little or no loss. Outputcouplers 540 may also allow light 550 to pass through with little loss.For example, in some implementations, output couplers 540 may have avery low diffraction efficiency for light 550 such that light 550 may berefracted or otherwise pass through output couplers 540 with littleloss, and thus may have a higher intensity than extracted light 560. Asa result, the user may be able to view combined images of theenvironment in front of combiner 515 and images of virtual objectsprojected by projector 510. In some implementations, output couplers 540may have a high diffraction efficiency for light 550 and may diffractlight 550 to certain desired directions (e.g., diffraction angles) withlittle loss.

In some embodiments, projector 510, input coupler 530, and outputcoupler 540 may be on any side of substrate 520. Input coupler 530 andoutput coupler 540 may be reflective gratings (also referred to asreflection gratings) or transmissive gratings (also referred to astransmission gratings) to couple display light into or out of substrate520.

FIG. 6 illustrates an example of an optical see-through augmentedreality system 600 including a waveguide display for exit pupilexpansion according to certain embodiments. Augmented reality system 600may be similar to augmented reality system 500, and may include thewaveguide display and a projector that may include a light source orimage source 612 and projector optics 614. The waveguide display mayinclude a substrate 630, an input coupler 640, and a plurality of outputcouplers 650 as described above with respect to augmented reality system500. While FIG. 5 only shows the propagation of light from a singlefield of view, FIG. 6 shows the propagation of light from multiplefields of view.

FIG. 6 shows that the exit pupil is replicated by output couplers 650 toform an aggregated exit pupil or eyebox, where different fields of view(e.g., different pixels on image source 612) may be associated withdifferent respective propagation directions towards the eyebox, andlight from a same field of view (e.g., a same pixel on image source 612)may have a same propagation direction for the different individual exitpupils. Thus, a single image of image source 612 may be formed by theuser's eye located anywhere in the eyebox, where light from differentindividual exit pupils and propagating in the same direction may be froma same pixel on image source 612 and may be focused onto a same locationon the retina of the user's eye. FIG. 6 shows that the image of theimage source is visible by the user's eye even if the user's eye movesto different locations in the eyebox.

In many waveguide-based near-eye display systems, in order to expand theeyebox of the waveguide-based near-eye display in two dimensions, two ormore output gratings may be used to expand the display light in twodimensions or along two axes (which may be referred to as dual-axispupil expansion). The two gratings may have different gratingparameters, such that one grating may be used to replicate the exitpupil in one direction and the other grating may be used to replicatethe exit pupil in another direction.

As described above, the input and output grating couplers describedabove can be volume holographic gratings or surface-relief gratings,which may have very different Klein-Cook parameter Q:

${Q = \frac{2{\pi\lambda}\; d}{n\; \Lambda^{2}}},$

where d is the thickness of the grating, λ is the wavelength of theincident light in free space, Λ is the grating period, and n is therefractive index of the recording medium. The Klein-Cook parameter Q maydivide light diffraction by gratings into three regimes. When a gratingis characterized by Q<<1, light diffraction by the grating may bereferred to as Raman-Nath diffraction, where multiple diffraction ordersmay occur for normal and/or oblique incident light. When a grating ischaracterized by Q>>1 (e.g., Q≥10), light diffraction by the grating maybe referred to as Bragg diffraction, where generally only the zeroth andthe ±1 diffraction orders may occur for light incident on the grating atan angle satisfying the Bragg condition. When a grating is characterizedby Q˜1, the diffraction by the grating may be between the Raman-Nathdiffraction and the Bragg diffraction. To meet Bragg conditions, thethickness d of the grating may be higher than certain values to occupy avolume (rather than at a surface) of a medium, and thus may be referredto as a volume Bragg grating. VBGs may generally have relatively smallrefractive index modulations (e.g., Δn≤0.05) and high spectral andangular selectivity, while surface-relief gratings may generally havelarge refractive index modulations (e.g., Δn≥0.5) and wide spectral andangular bandwidths.

FIG. 7A illustrates the spectral bandwidth of an example of a volumeBragg grating (e.g., a reflective VBG) and the spectral bandwidth of anexample of a surface-relief grating (e.g., a transmissive SRG). Thehorizontal axis represents the wavelength of the incident visible lightand the vertical axis corresponds to the diffraction efficiency. Asshown by a curve 710, the diffraction efficiency of the reflective VBGis high in a narrow wavelength range, such as green light. In contrast,the diffraction efficiency of the transmissive SRG may be high in a verywide wavelength range, such as from blue to red light, as shown by acurve 720.

FIG. 7B illustrates the angular bandwidth of an example of a volumeBragg grating (e.g., a reflective VBG) and the angular bandwidth of anexample of a surface-relief grating (e.g., a transmissive SRG). Thehorizontal axis represents the incident angle of the visible lightincident on the grating, and the vertical axis corresponds to thediffraction efficiency. As shown by a curve 715, the diffractionefficiency of the reflective VBG is high for light incident on thegrating from a narrow angular range, such as about ±2.5° from theperfect Bragg condition. In contrast, the diffraction efficiency of thetransmissive SRG is high in a very wide angular range, such as greaterthan about ±10° or wider, as shown by a curve 725.

Due to the high spectral selectivity at the Bragg condition, VBGs, suchas reflective VBGs, may allow for single-waveguide design withoutcrosstalk between primary colors, and may exhibit superior see-throughquality. However, the spectral and angular selectivity may lead to lowerefficiency because only a portion of the display light in the full FOVmay be diffracted and reach user's eyes.

FIG. 8A illustrates an example of an optical see-through augmentedreality system including a waveguide display 800 and surface-reliefgratings for exit pupil expansion according to certain embodiments.Waveguide display 800 may include a substrate 810 (e.g., a waveguide),which may be similar to substrate 520. Substrate 810 may be transparentto visible light and may include, for example, a glass, quartz, plastic,polymer, PMMA, ceramic, or crystal substrate. Substrate 810 may be aflat substrate or a curved substrate. Substrate 810 may include a firstsurface 812 and a second surface 814. Display light may be coupled intosubstrate 810 by an input coupler 820, and may be reflected by firstsurface 812 and second surface 814 through total internal reflection,such that the display light may propagate within substrate 810. Asdescribed above, input coupler 820 may include a grating, a refractivecoupler (e.g., a wedge or a prism), or a reflective coupler (e.g., areflective surface having a slant angle with respect to substrate 810).For example, in one embodiment, input coupler 820 may include a prismthat may couple display light of different colors into substrate 810 ata same refraction angle. In another example, input coupler 820 mayinclude a grating coupler that may diffract light of different colorsinto substrate 810 at different directions. Input coupler 820 may have acoupling efficiency of greater than 10%, 20%, 30%, 50%, 75%, 90%, orhigher for visible light.

Waveguide display 800 may also include a first grating 830 and a secondgrating 840 positioned on one or two surfaces (e.g., first surface 812and second surface 814) of substrate 810 for expanding incident displaylight beam in two dimensions in order to fill an eyebox 850 (or outputor exit pupil) with the display light. First grating 830 may beconfigured to expand at least a portion of the display light beam alongone direction, such as approximately in the x direction. Display lightcoupled into substrate 810 may propagate in a direction shown by a line832. While the display light propagates within substrate 810 along adirection shown by line 832, a portion of the display light may bediffracted by a portion of first grating 830 towards second grating 840as shown by a line 834 each time the display light propagating withinsubstrate 810 reaches first grating 830. Second grating 840 may thenexpand the display light from first grating 830 in a different direction(e.g., approximately in the y direction) by diffracting a portion of thedisplay light to eyebox 850 each time the display light propagatingwithin substrate 810 reaches second grating 840.

FIG. 8B illustrates an example of an eye box including two-dimensionalreplicated exit pupils. FIG. 8B shows that a single input pupil 805 maybe replicated by first grating 830 and second grating 840 to form anaggregated exit pupil 860 that includes a two-dimensional array ofindividual exit pupils 852. For example, the exit pupil may bereplicated in approximately the x direction by first grating 830 and inapproximately the y direction by second grating 840. As described above,output light from individual exit pupils 852 and propagating in a samedirection may be focused onto a same location in the retina of theuser's eye. Thus, a single image may be formed by the user's eye fromthe output light in the two-dimensional array of individual exit pupils852.

FIG. 9A illustrates wave vectors of light diffracted by examples ofsurface-relief gratings for exit pupil expansion in a waveguide displayand exit pupils for multiple colors. A circle 910 may represent wavevectors of light that may be guided by the waveguide. For light withwave vectors outside of circle 910, the light may become evanescent. Acircle 920 may represent wave vectors of light that may leak out of thewaveguide because the total-internal-reflection condition is not met.Thus, the ring between circle 910 and circle 920 may represent the wavevectors of light that can be guided by the waveguide and can propagatewithin the waveguide through TIR. Wave vectors 932 show the lightdispersion caused by the input grating, where light of different colorsmay have different wave vectors and different diffraction angles. Wavevectors 942 show the light dispersion caused by a front grating (e.g.,first grating 830), where light of different colors may have differentdiffraction angles. Wave vectors 952 show the light dispersion caused bya back grating (e.g., second grating 840), where light of differentcolors may have different diffraction angles. The wave vectors for eachcolor may form a respective closed triangle, and the triangles fordifferent colors may share a common origin vertex 922. Thus, the overalldispersion by the three gratings may be close to zero.

Even though the overall dispersion by the three gratings may be zero,the dispersion by each grating may cause the reduction or clipping ofthe field of view of the waveguide display due to the conditions underwhich light may be guided by the waveguide as shown by the ring betweencircle 910 and circle 920. For example, for a FOV 924, the footprints ofthe FOV after the diffraction by the input grating may be different fordifferent colors due to the dispersion by the input grating. In theexample shown in FIG. 9A, a footprint 936 of the FOV for light of afirst color may be located in the ring, while a portion of a footprint934 of the FOV for light of a second color and a portion of a footprint938 of the FOV for light of a third color may fall outside of the ringand thus may not be guided by the waveguide. In addition, the footprintsof the FOV after the diffraction by the front grating may be furtherclipped or reduced. In the example shown in FIG. 9A, a small portion ofa footprint 946 of the FOV for the light of the first color, a largeportion of a footprint 944 of the FOV for the light of the second color,and a large portion of a footprint 948 of the FOV for the light of thethird color may fall outside of the ring and thus may not be guided bythe waveguide and diffracted by the back grating to reach the exitpupil.

FIG. 9B illustrates the field-of-view clipping by the examples ofsurface-relief gratings for exit pupil expansion in the waveguidedisplay. For example, the FOV for the light of the first color after thediffraction by the back grating may be shown by a footprint 956, whichmay be close to the full FOV. For the light of the second color, a topportion of the FOV may be clipped after diffraction by the first gratingand a right portion of the FOV may be clipped after diffraction by thefront grating. Thus, the FOV for the light of the second color after thediffraction by the back grating may be shown by a footprint 954, whichmay be much smaller than the full FOV. Similarly, for the light of thethird color, a bottom portion of the FOV may be clipped afterdiffraction by the first grating and a left portion of the FOV may beclipped after diffraction by the front grating. Thus, the FOV for thelight of the third color after the diffraction by the back grating maybe shown by a footprint 958, which may be much smaller than the fullFOV. Thus, certain color components of the image may be missing forcertain fields of view. As such, in order to achieve the full FOV fordifferent colors, two or more waveguides and the corresponding gratingsmay be used. In addition, as described above, the wide bandwidth of SRGsmay cause crosstalk between light of different primary colors and/orfrom different FOVs, and thus multiple waveguides may also be used toavoid the crosstalk.

Due to the high spectral selectivity at the Bragg condition, VBGs, suchas reflective VBGs, may allow for single-waveguide design withoutcrosstalk between primary colors in a volume Bragg grating and mayachieve a superior see-through quality. Thus, input coupler 530 or 640and output coupler 540 or 650 may include a volume Bragg grating, whichmay be a volume hologram recorded in a holographic recording material byexposing the holographic recording material to light patterns generatedby the interference between two or more coherent light beams. In volumeBragg gratings, the incident angle and the wavelength of the incidentlight may need to satisfy the Bragg phase-matching condition in orderfor the incident light to be diffracted by the Bragg grating. When asingle Bragg grating is used in a waveguide-based near-eye display, thespectral and angular selectivity of the volume Bragg gratings may leadto lower efficiency because only a portion of the display light may bediffracted and reach user's eyes, and the field of view and the workingwavelength range of the waveguide-based near-eye display may be limited.In some embodiments, multiplexed VBGs may be used to improve theefficiency and increase the FOV.

FIG. 10A illustrates the front view of an example of a volume Bragggrating-based waveguide display 1000 according to certain embodiments.Waveguide display 1000 may include a substrate 1010, which may besimilar to substrate 520. Substrate 1010 may be transparent to visiblelight and may include, for example, a glass, quartz, plastic, polymer,PMMA, ceramic, or crystal substrate. Substrate 1010 may be a flatsubstrate or a curved substrate. Substrate 1010 may include a firstsurface 1012 and a second surface 1014. Display light may be coupledinto substrate 1010 by an input coupler 1020, and may be reflected byfirst surface 1012 and second surface 1014 through total internalreflection, such that the display light may propagate within substrate1010. As described above, input coupler 1020 may include a diffractivecoupler (e.g., a volume holographic grating or a surface-reliefgrating), a refractive coupler (e.g., a wedge or a prism), or areflective coupler (e.g., a reflective surface having a slant angle withrespect to substrate 1010). For example, in one embodiment, inputcoupler 1020 may include a prism that may couple display light ofdifferent colors into substrate 1010 at a same refraction angle. Inanother example, the input coupler may include a grating coupler thatmay diffract light of different colors into substrate 1010 at differentdirections.

Waveguide display 1000 may also include a first grating 1030 and asecond grating 1040 positioned on one or two surfaces (e.g., firstsurface 1012 and second surface 1014) of substrate 1010 for expandingincident display light beam in two dimensions in order to fill an eyebox1050 with the display light. First grating 1030 may include one or moremultiplexed volume Bragg gratings each configured to expand at least aportion of the display light beam (e.g., light corresponding to acertain field of view and/or a wavelength range) along one direction, asshown by lines 1032, 1034, and 1036. For example, while the displaylight propagates within substrate 1010 along a direction shown by line1032, 1034, or 1036, a portion of the display light may be diffracted byfirst grating 1030 to second grating 1040 each time the display lightpropagating within substrate 1010 reaches first grating 1030. Secondgrating 1040 may then expand the display light from first grating 1030in a different direction by diffracting a portion of the display lightto eyebox 1050 each time the display light propagating within substrate1010 reaches second grating 1040.

As described above, first grating 1030 and second grating 1040 may eachinclude a multiplexed VBG that includes multiple VBGs each designed fora specific FOV range and/or wavelength range. For example, first grating1030 may include a few hundred or more VBGs (e.g., about 300 to about1000 VBGs) recorded by a few hundred or more exposures, where each VBGmay be recorded under a different condition. Second grating 1040 mayalso include tens or hundreds of VBGs (e.g., 50 or more VBGs) recordedby tens or hundreds of exposures. First grating 1030 and second grating1040 may each be a transmission grating or a reflection grating.

FIGS. 10B and 10C illustrate the top and side views of volume Bragggrating-based waveguide display 1000, respectively. Input coupler 1020may include projector optics (not shown, e.g., a lens) and a prism.Display light may be collimated and projected onto the prism by theprojector optics, and may be coupled into substrate 1010 by the prism.The prism may have a refractive index that matches the refractive indexof substrate 1010 and may include a wedge having a certain angle suchthat light coupled into substrate 1010 may be incident on surface 1012or 1014 of substrate 1010 at an incident angle greater than the criticalangle for substrate 1010. As such, display light coupled into substrate1010 may be guided by substrate 1010 through total internal reflection,and may be diffracted by multiple regions of first grating 1030 towardssecond grating 1040 as described above. Second grating 1040 may thendiffract the display light out of substrate 1010 at multiple regions toreplicate the exit pupil.

FIG. 11 illustrates light dispersion in an example of a volume Bragggrating-based waveguide display, such as waveguide display 1000,according to certain embodiments. As shown in the example, a sphere 1110may represent wave vectors of light that may be guided by the waveguide.For light with wave vectors outside of sphere 1110, the light may becomeevanescent. A cone 1120 may represent wave vectors of light that mayleak out of the waveguide because the total-internal-reflectioncondition is not met. Thus, the region of sphere 1110 outside of cone1120 may represent the wave vectors of light that can be guided by thewaveguide and can propagate within the waveguide through TIR. Point 1130may represent the wave vector of the display light coupled into thewaveguide by, for example, a prism. Wave vectors 1140 show the lightdispersion caused by first grating 1030, where light of different colorsmay have different diffraction angles. Wave vectors 1150 show the lightdispersion caused by second grating 1040, where light of differentcolors may have different diffraction angles. Thus, the light coupledout of the substrate may have some dispersion, such that the images ofdifferent colors may not perfectly overlap with each other to form oneimage. Therefore, the displayed image may be blurred and the resolutionof the displayed image may be reduced.

FIG. 12A illustrates an example of a volume Bragg grating 1200. VolumeBragg grating 1200 shown in FIG. 12A may include a transmissionholographic grating that has a thickness D. The refractive index n ofvolume Bragg grating 1200 may be modulated at an amplitude Δn, and thegrating period of volume Bragg grating 1200 may be Λ. Incident light1210 having a wavelength λ may be incident on volume Bragg grating 1200at an incident angle θ, and may be refracted into volume Bragg grating1200 as incident light 1220 that propagates at an angle θ_(n) in volumeBragg grating 1200. Incident light 1220 may be diffracted by volumeBragg grating 1200 into diffraction light 1230, which may propagate at adiffraction angle &d in volume Bragg grating 1200 and may be refractedout of volume Bragg grating 1200 as diffraction light 1240.

FIG. 12B illustrates the Bragg condition for volume Bragg grating 1200shown in FIG. 12A. Volume Bragg grating 1200 may be a transmissivegrating. A vector 1205 may represent the grating vector {right arrowover (G)}, where |{right arrow over (G)}|=2π/Λ. A vector 1225 mayrepresent the incident wave vector {right arrow over (k_(l))}, and avector 1235 may represent the diffract wave vector {right arrow over(k_(d))}, where |{right arrow over (k_(l))}|=|{right arrow over(k_(d))}=2π/λ. Under the Bragg phase-matching condition, {right arrowover (k)}_(l)-{right arrow over (k_(d))}={right arrow over (G)}. Thus,for a given wavelength λ, there may only be one pair of incident angle θ(or θ_(n)) and diffraction angle θ_(d) that meets the Bragg conditionperfectly. Similarly, for a given incident angle θ, there may be onewavelength λ that meets the Bragg condition perfectly. As such, thediffraction may occur for a small wavelength range and in a smallincident angular range around a perfect Bragg condition. The diffractionefficiency, the wavelength selectivity, and the angular selectivity ofvolume Bragg grating 1200 may be functions of thickness D of volumeBragg grating 1200. For example, the full-width-half-magnitude (FWHM)wavelength range and the FWHM angular range of volume Bragg grating 1200around the Bragg condition may be inversely proportional to thickness Dof volume Bragg grating 1200, while the maximum diffraction efficiencyat the Bragg condition may be a function of sin²(α×Δn×D), where α is acoefficient. For a reflective volume Bragg grating, the maximumdiffraction efficiency at the Bragg condition may be a function of tanh²(α×Δn×D).

As described above, in some designs, in order to achieve a large FOV(e.g., larger than) ±30° and diffract light of different colors,multiple polymer layers each including a Bragg grating for a differentcolor (e.g., R, G, or B) and/or a different FOV may be arranged in astack for coupling the display light to the user's eyes. In somedesigns, a multiplexed Bragg grating may be used, where each part of themultiplexed Bragg grating may be used to diffract light in a differentFOV range and/or within a different wavelength range. Thus, in somedesigns, in order to achieve a desired diffraction efficiency and alarge FOV for the full visible spectrum (e.g., from about 400 nm toabout 700 nm, or from about 450 nm to about 650 nm), one or more thickvolume Bragg gratings each including a large number of gratings (orholograms) recorded by a large number of exposures (e.g., holographicrecordings), such as a few hundred or more than 1000, may be used.

VBGs or other holographic optical elements described above may berecorded in a holographic material (e.g., photopolymer) layer. In someembodiments, the VBGs can be recorded first and then laminated on asubstrate in a near-eye display system. In some embodiments, aholographic material layer may be coated or laminated on the substrateand the VBGs may then be recorded in the holographic material layer.

In general, to record a holographic optical element in a photosensitivematerial layer, two coherent beams may interfere with each other atcertain angles to generate a unique interference pattern in thephotosensitive material layer, which may in turn generate a uniquerefractive index modulation pattern in the photosensitive materiallayer, where the refractive index modulation pattern may correspond tothe light intensity pattern of the interference pattern. Thephotosensitive material layer may include, for example, silver halideemulsion, dichromated gelatin, photopolymers includingphoto-polymerizable monomers suspended in a polymer matrix,photorefractive crystals, and the like. One example of thephotosensitive material layer for holographic recording is two-stagephotopolymers that may include matrix precursors that can be pre-curedto form polymeric binders before holographic recording and writingmonomers for holographic recording.

In one example, the photosensitive material layer may include polymericbinders, monomers (e.g., acrylic monomers), and initiating agents, suchas initiators, chain transfer agents, or photosensitizing dyes. Thepolymeric binders may act as the support matrix. The monomers may bedispersed in the support matrix and may serve as refractive indexmodulators. The photosensitizing dyes may absorb light and interact withthe initiators to polymerize the monomers. Thus, in each exposure(recording), the interference pattern may cause the polymerization anddiffusion of the monomers to bright fringes, thus generatingconcentration and density gradients that may result in refractive indexmodulation. For example, areas with a higher concentration of monomersand polymerization may have a higher refractive index. As the exposureand polymerization proceed, fewer monomers may be available forpolymerization, and the diffusion may be suppressed. After all orsubstantially all monomers have been polymerized, no more new gratingsmay be recorded in the photosensitive material layer. In a thick VBGthat includes a large number of gratings recorded in a large number ofexposures, display haze may be significant.

As described above, in some waveguide-based near-eye display systems, inorder to expand the eyebox of the waveguide-based near-eye display, twooutput gratings (or two grating layers or two portions of a multiplexedgrating) may generally be used to expand the display light in twodimensions or along two axes for dual-axis pupil expansion. Spatiallyseparating the two output gratings and reducing the total number ofexposures for each output grating may help to reduce the display hazebecause the see-through region (e.g., the middle) of the waveguide-basednear-eye display may only include one output grating. For example, insome embodiments, the first output grating may be recorded with moreexposures (e.g., >500 or >1000 times) and may be positioned outside ofthe see-through region of the waveguide-based near-eye display. Thesecond output grating may be recorded with fewer exposures (e.g., <100or <50 times) and may be positioned in the see-through region of thewaveguide-based near-eye display. Thus, the display haze in thesee-through region may be significantly reduced. However, because of thespatial separation of the two output gratings, the overall size of thewaveguide-based near-eye display can be very large.

The grating couplers described above may include transmissive VBGs orreflective VBGs, which may have some similar and some differentcharacteristics. For example, as described above, thefull-width-half-magnitude (FWHM) wavelength range and the FWHM angularrange of a transmissive or reflective volume Bragg grating near theBragg condition may be inversely proportional to thickness D of thetransmissive or reflective volume Bragg grating. The maximum diffractionefficiency at the Bragg condition for a transmissive VBG may be afunction of sin²(α×Δn×D), where α is a coefficient and Δn is therefractive index modulation, while the maximum diffraction efficiency atthe Bragg condition for a reflective VBG may be a function of tanh²(α×Δn×D). In addition, the parameters (e.g., the grating tilt angles)of the transmissive and reflective volume Bragg gratings may bedifferent in order to couple the display light into the waveguide atcertain angles such that the coupled display light can be guided by thewaveguide through TIR. Because of the different grating parameters, thedispersion characteristics of transmissive gratings and reflectivegratings may be different.

FIG. 13A illustrates an example of a reflective volume Bragg grating1300 in a waveguide display according to certain embodiments. Thegrating tilt angle α of reflective VBG 1300 may need to be within acertain range to reflectively diffract the display light. If the gratingtilt angle α of reflective VBG 1300 is greater than a certain value,reflective VBG 1300 may become a transmissive VBG, the distance betweentwo consecutive locations where the display light may reach the gratingmay be too large (and thus the exit pupil may be sparsely replicated inthe eyebox), or the display light may become evanescent. In one example,the grating tilt angle α of reflective VBG 1300 may be about 30°.

FIG. 13B illustrates an example of a reflective VBG 1310 in a waveguidedisplay where light diffracted by the reflective VBG is not totallyreflected and guided in the waveguide. The grating tilt angle α ofreflective VBG 1310 shown in FIG. 13B may be less than a certain value.As such, light coupled into the waveguide may be incident on the surfaceof the waveguide at an incident angle less than the critical angle, andthus may not be totally reflected and guided in the waveguide. Thegrating tilt angle α of reflective VBG 1310 may be less than about 30°.Thus, the grating tilt angle α of a reflective VBG may need to be withina certain range to reflectively diffract the display light into thewaveguide such that the diffracted light may be guided by the waveguidethrough total internal reflection.

FIG. 13C illustrates an example of a transmissive volume Bragg grating1350 in a waveguide display according to certain embodiments. Thegrating tilt angle α of transmissive VBG 1350 may also need to be withina certain range. For example, if the grating tilt angle α oftransmissive VBG 1350 is lower than a certain value, transmissive VBG1350 may become a reflective VBG, the distance between two consecutivelocations where the display light may reach the grating may be too large(and thus the exit pupil may be sparsely replicated in the eyebox), orthe display light may become evanescent.

FIG. 13D illustrates an example of a transmissive VBG 1360 in awaveguide display where light diffracted by the transmissive VBG is nottotally reflected and guided in the waveguide. The grating tilt angle αof transmissive VBG 1360 may be greater than a certain value, such asgreater than about 60°. As such, light coupled into the waveguide may beincident on the surface of the waveguide at an incident angle less thanthe critical angle, and thus may not be totally reflected and guided inthe waveguide. Thus, the grating tilt angle α of a transmissive VBG mayneed to be within a certain range to transmissively diffract the displaylight into the waveguide such that the diffracted light may be guided bythe waveguide through total internal reflection. FIGS. 13A-13D show thatthe grating tilt angle α may be smaller for reflective gratings than fortransmissive gratings.

FIG. 14A illustrates the light dispersion by an example of a reflectivevolume Bragg grating 1400 in a waveguide display according to certainembodiments. Reflective VBG 1400 may be characterized by a gratingvector k_(g), a thickness d, and an average refractive index n. Thesurface normal direction of reflective VBG 1400 is N. The amount oflight dispersion by reflective VBG 1400 may be determined by:

${{\Delta\theta} = \frac{\lambda_{0}{{k_{g} \times N}}}{n \times d{{k_{g} \cdot k_{out}}}}},$

where λ₀ is the wavelength of the light that perfectly meets the Braggcondition, and k_(out) is the wave vector of the light diffracted byreflective VBG 1400. When the grating tilt angle α of reflective VBG1400 is about 30°, the amount of light dispersion by reflective VBG 1400may be approximately:

${{\Delta\theta} \propto \frac{\sin \; 30{^\circ}}{d \times \cos \; 30{^\circ}}} = {\frac{0.58}{d}.}$

Thus, to achieve an angular resolution about 2 arcminutes, the thicknessd of reflective VBG 1400 may be at least about 0.5 mm.

FIG. 14B illustrates the light dispersion by an example of atransmissive volume Bragg grating 1450 in a waveguide display accordingto certain embodiments. Transmissive VBG 1450 may similarly becharacterized by a grating vector k_(g), a thickness d, and an averagerefractive index n. The surface normal direction of transmissive VBG1450 is N. The amount of light dispersion by transmissive VBG 1450 maybe determined by:

${{\Delta\theta} = \frac{\lambda_{0}{{k_{g} \times N}}}{n \times d{{k_{g} \cdot k_{out}}}}},$

where λ₀ is the wavelength of the light that perfectly meets the Braggcondition, and k_(out) is the wave vector of the light diffracted bytransmissive VBG 1450. When the grating tilt angle α of transmissive VBG1450 is about 60°, the amount of light dispersion by transmissive VBG1450 may be approximately:

${{\Delta\theta} \propto \frac{\sin \; 60{^\circ}}{d \times \cos \; 60{^\circ}}} = {\frac{1.73}{d}.}$

Thus, to achieve an angular resolution about 2 arcminutes, the thicknessd of transmissive VBG 1450 may be at least about 1.5 mm, which is aboutthree times of the thickness of a reflective VBG with the same angularresolution and may be difficult to achieve or may cause significantdisplay haze.

In order to reduce the thickness of the VBGs and display haze andachieve the desired resolution, dispersion compensation may be desiredin a VBG-based waveguide display. According to certain embodiments, oneor more pairs of gratings having matching grating vectors and operatingin opposite diffraction conditions (e.g., +1 order diffraction versus −1order diffraction) may be used to compensate for the dispersion causedby each other.

FIGS. 15A-15B illustrates front and side views of an example of a volumeBragg grating-based waveguide display 1500 with exit pupil expansion anddispersion reduction according to certain embodiments. Waveguide display1500 may be similar to waveguide display 1000, and may include an inputcoupler 1520 at a different location compared with input coupler 1020.Waveguide display 1500 may include a substrate 1510, and a first grating1530 and a second grating 1540 on substrate 1510. As input coupler 1020,input coupler 1520 may include projector optics 1522 (e.g., a lens) anda prism 1524. Display light may be coupled into substrate 1510 by inputcoupler 1520 and may be guided by substrate 1510. The display light mayreach a first portion 1532 of first grating 1530 and may be diffractedby first portion 1532 of first grating 1530 to change the propagationdirection and reach other portions of first grating 1530, which may eachdiffract the display light towards second grating 1540. Second grating1540 may diffract the display light out of substrate 1510 at differentlocations to form multiple exit pupils as described above.

First portion 1532 and each of other portions of first grating 1530 mayhave matching grating vectors (e.g., having a same grating vector in thex-y plane and having a same grating vector, opposite grating vectors, orboth the same and opposite grating vectors in the z direction, butrecorded in different exposure durations to achieve differentdiffraction efficiencies). Therefore, they may compensate for thedispersion of display light caused by each other to reduce the overalldispersion, due to the opposite Bragg conditions (e.g., +1 order and −1order diffractions) for the diffractions at first portion 1532 and eachof other portions of first grating 1530. Therefore, the overalldispersion of the display light by waveguide display 1500 may be reducedin at least one direction.

FIG. 16A is a front view of an example of a volume Bragg grating-basedwaveguide display 1600 with exit pupil expansion and dispersionreduction according to certain embodiments. FIG. 16B is a side view ofthe example of volume Bragg grating-based waveguide display 1600 withexit pupil expansion and dispersion reduction according to certainembodiments. Waveguide display 1600 may be similar to waveguide display1500, but may include an input coupler that is different from inputcoupler 1520. Waveguide display 1600 may include a substrate 1610 and afirst grating 1630 and a second grating 1640 on substrate 1610. Theinput coupler may include projector optics 1620 (e.g., a lens) and ainput grating 1622, rather than a prism. Display light may be collimatedby projector optics 1620 and projected onto input grating 1622, whichmay couple the display light into substrate 1610 by diffraction asdescribed above with respect to, for example, FIGS. 5 and 6. The displaylight may reach a first portion 1632 of first grating 1630 and may bediffracted by first portion 1632 of first grating 1630 to change thepropagation direction and reach other portions of first grating 1630,which may each diffract the display light towards second grating 1640.Second grating 1640 may diffract the display light out of substrate 1610at different locations to form multiple exit pupils as described above.

First portion 1632 and each of other portions of first grating 1630 mayhave matching grating vectors (e.g., having a same grating vector in thex-y plane and a same grating vector and/or opposite grating vectors inthe z direction, but recorded in different exposure durations to achievedifferent diffraction efficiencies). Therefore, they may compensate forthe dispersion of display light caused by each other to reduce theoverall dispersion in one direction, due to the opposite Braggconditions (e.g., +1 order and −1 order diffractions) for thediffractions at first portion 1632 and each of other portions of firstgrating 1630. In addition, input grating 1622 and second grating 1640may have matching grating vectors (e.g., having the same grating vectorin the x-y plane and having the same or opposite grating vectors in thez direction, but recorded in different exposure durations to achievedifferent diffraction efficiencies), where input grating 1622 may couplethe display light into substrate 1610, while second grating 1640 maycouple the display light out of the waveguide. Therefore, input grating1622 and second grating 1640 may compensate for the dispersion ofdisplay light caused by each other to reduce the overall dispersion inat least one direction, due to the opposite diffraction directions andopposite Bragg conditions (e.g., +1 order and −1 order diffractions) forthe diffractions at input grating 1622 and second grating 1640. In thisway, the dispersion by first portion 1632 and each of other portions offirst grating 1630 may be canceled out, and the dispersion by inputgrating 1622 and second grating 1640 may also be canceled out.Therefore, the overall dispersion of the display light by waveguidedisplay 1600 can be minimized in any direction. As such, a higherresolution of the displayed image may be achieved.

Thus, thinner reflective or transmissive VBGs may be used as the inputand output couplers and may still achieve the desired resolution.Transmissive VBGs may also allow the first and second gratings to be atleast partially overlapped to reduce the physical dimensions of thewaveguide display as described in detail below.

FIG. 17A illustrates the propagation of light from different fields ofview in a reflective volume Bragg grating-based waveguide display 1700according to certain embodiments. Waveguide display 1700 may include areflective VBG 1710. Due to the grating tilt angle and thus the gratingvector of reflective VBG 1710, light from a positive field of view(shown by a line 1722) may have a smaller incident angle on fringes ofreflective VBG 1710 and also a smaller incident angle on the top surface1702 of waveguide display 1700. On the other hand, light from a negativefield of view (shown by a line 1724) may have a larger incident angle onthe fringes of reflective VBG 1710 and also a larger incident angle ontop surface 1702 of waveguide display 1700.

FIG. 17B illustrates the propagation of light from different fields ofview in a transmissive volume Bragg grating-based waveguide display 1750according to certain embodiments. Waveguide display 1750 may include atransmissive VBG 1760. Due to the grating tilt angle differences,transmissive VBG 1760 may diffract light from different fields of viewin different manners compared with reflective VBG 1710. For example, asillustrated, light from a positive field of view (shown by a line 1772)may have a smaller incident angle on fringes of transmissive VBG 1760but a larger incident angle on the bottom surface 1752 of waveguidedisplay 1750. On the other hand, light from a negative field of view(shown by a line 1774) may have a larger incident angle on the fringesof transmissive VBG 1760 but a smaller incident angle on the bottomsurface 1752 of waveguide display 1750. The manner of diffraction oflight from different fields of view by a grating may affect the formfactor of the waveguide display.

FIG. 18 illustrates an example of a reflective volume Bragggrating-based waveguide display 1800 with exit pupil expansion anddispersion reduction according to certain embodiments. Waveguide display1800 may include a top grating 1805 and a bottom grating 1815. In theillustrated example, top grating 1805 may be a reflective VBG, andbottom grating 1815 may also be a reflective grating. On bottom grating1815, an exit region 1850 represents the region where display light forthe full FOV at one pupil location in the eyebox (e.g., at the centerthe eyebox) may be coupled out of the bottom grating. As shown in FIG.18, the top FOV of exit region 1850 represented by a line between a topright corner 1822 and a top left corner 1824 may map to a curve 1830 ontop grating 1805, where top right corner 1822 and top left corner 1824of exit region 1850 may map to a location 1832 and a location 1834 ontop grating 1805, respectively. The bottom FOV of exit region 1850represented by a line between a bottom right corner 1842 and a bottomleft corner 1844 may map to a curve 1810 on top grating 1805, wherebottom right corner 1842 and bottom left corner 1844 of exit region 1850may map to a location 1812 and a location 1814 on top grating 1805,respectively. Thus, if curve 1830 is below the line between top rightcorner 1822 and top left corner 1824 of exit region 1850, there may besome FOV clipping. As such, to preserve the full FOV, curve 1830 may beabove the line between top right corner 1822 and top left corner 1824 ofexit region 1850. Therefore, the size of waveguide display 1800 may belarge.

FIG. 19 illustrates an example of a transmissive volume Bragggrating-based waveguide display 1900 with exit pupil expansion andform-factor reduction according to certain embodiments. Waveguidedisplay 1900 may include a top grating 1905 and a bottom grating 1915.In the illustrated example, top grating 1905 may be a reflective VBG,and bottom grating 1915 may be a transmission grating. On bottom grating1915, an exit region 1950 represents the region where display light forthe full FOV at one pupil location in the eyebox (e.g., at the centerthe eyebox) may be coupled out of the bottom grating. As shown in FIG.19, the top FOV of exit region 1950 represented by a line between a topright corner 1922 and a top left corner 1924 may map to a curve 1910 ontop grating 1905, where top right corner 1922 and top left corner 1924of exit region 1950 may map to a location 1912 and a location 1914 ontop grating 1905, respectively. The bottom FOV of exit region 1950represented by a line between a bottom right corner 1942 and a bottomleft corner 1944 may map to a curve 1930 on top grating 1905, wherebottom right corner 1942 and bottom left corner 1944 of exit region 1950may map to a location 1932 and a location 1934 on top grating 1905,respectively. Thus, there can be some overlap between top grating 1905and bottom grating 1915 to reduce the overall size of waveguide display1900. For example, location 1932 may be lower than top right corner 1922and can still be mapped to bottom right corner 1942.

FIG. 20 illustrates another example of a transmissive volume Bragggrating-based waveguide display 2000 with an image projector 2030according to certain embodiments. Waveguide display 2000 may include atop grating 2005 and a bottom grating 2015. Top grating 2005 may includea reflective VBG, and bottom grating 2015 may include a transmissiveVBG. The exit region on bottom grating 2015 supporting the desired fieldof view of waveguide display 2000 at one pupil location in the eyebox(e.g., in the center of the eyebox) is represented by an octagon 2020. Ashape 2010 represents the mapping of the FOV shown by octagon 2020 tothe region on top grating 2005. As described above with respect to FIG.19, because bottom grating 2015 is a transmission grating, top grating2005 and bottom grating 2015 may at least partially overlap to reducethe physical size of waveguide display 2000.

FIG. 21 illustrates an example of a volume Bragg grating-based waveguidedisplay 2100 with exit pupil expansion, dispersion reduction, andform-factor reduction according to certain embodiments. Waveguidedisplay 2100 may include a substrate 2110, which may be similar tosubstrate 1610 but may be much smaller than substrate 1610. Substrate2110 may include a first surface 2112 and a second surface 2114. Displaylight from a light source (e.g., LEDs) may be coupled into substrate2110 by an input coupler 2120, and may be reflected by first surface2112 and second surface 2114 through total internal reflection, suchthat the display light may propagate within substrate 2110. Inputcoupler 2120 may include a diffractive coupler (e.g., a volumeholographic grating) and may couple display light of different colorsinto substrate 2110 at different diffraction angles.

As waveguide display 1600, waveguide display 2100 may also include afirst grating 2130 and a second grating 2140 formed on first surface2112 and/or second surface 2114. For example, first grating 2130 andsecond grating 2140 may be formed on a same surface or two differentsurface of substrate 2110. Second grating 2140 may be formed in thesee-through region of the waveguide display and may overlap with aneyebox 2150 when viewed in the z direction (e.g., at a distance about 18mm from second grating 2140 in +z or −z direction). First grating 2130and second grating 2140 may be used for dual-axis pupil expansion toexpand the incident display light beam in two dimensions to fill eyebox2150 with the display light. First grating 2130 may be a transmissiongrating or a reflection grating. Second grating 2140 may include atransmission grating to at least partially overlap with first grating2130 and reduce the form factor of waveguide display 2100 as describedbelow.

In addition, waveguide display 2100 may also include a third grating2160 formed on first surface 2112 or second surface 2114. In someembodiments, third grating 2160 and first grating 2130 may be on a samesurface of substrate 2110. In some embodiments, third grating 2160 andfirst grating 2130 may be in different regions of a same grating or asame grating material layer as shown in FIG. 16. In some embodiments,third grating 2160 may be spatially separate from first grating 2130. Insome embodiments, third grating 2160 and first grating 2130 may berecorded in a same number of exposures and under similar recordingconditions (but may be recorded for different exposure durations toachieve different diffraction efficiencies), such that each VBG in thirdgrating 2160 may match a respective VBG in first grating 2130 (e.g.,having the same grating vector in the x-y plane and having the sameand/or opposite grating vectors in the z direction). For example, insome embodiments, a VBG in third grating 2160 and a corresponding VBG infirst grating 2130 may have the same grating period and the same gratingslant angle (and thus the same grating vector), and the same thickness.In one embodiment, third grating 2160 and first grating 2130 may have athickness about 20 μm and may each include about 40 or more VBGsrecorded through about 40 or more exposures. In some embodiments, secondgrating 2140 may have a thickness about 20 μm or higher, and may includeabout 50 or more VBGs recorded through about 50 or more exposures.

Input coupler 2120 may couple the display light from the light sourceinto substrate 2110. The display light may reach third grating 2160directly or may be reflected by first surface 2112 and/or second surface2114 to third grating 2160, where the size of the display light beam maybe slightly larger than that at input coupler 2120. Each VBG in thirdgrating 2160 may diffract a portion of the display light within a FOVrange and a wavelength range that approximately satisfies the Braggcondition of the VBG to first grating 2130. While the display lightdiffracted by a VBG in third grating 2160 propagates within substrate2110 (e.g., along a direction shown by a line 2132) through totalinternal reflection, a portion of the display light may be diffracted bythe corresponding VBG in first grating 2130 to second grating 2140 eachtime the display light propagating within substrate 2110 reaches firstgrating 2130. Second grating 2140 may then expand the display light fromfirst grating 2130 in a different direction by diffracting a portion ofthe display light to eyebox 2150 each time the display light propagatingwithin substrate 2110 reaches second grating 2140.

Because third grating 2160 and first grating 2130 may be thin (e.g.,about 20 μm), they may cause some dispersion, but the dispersion may notbe as high as the dispersion of a grating having a thickness of, forexample, 1 μm or thinner. Therefore, the fields of view for differentcolors may not be significantly affected by the dispersion. In addition,as described above, each VBG in third grating 2160 matches a respectiveVBG in first grating 2130 (e.g., having the same grating vector in thex-y plane and having the same and/or opposite grating vector in the zdirection), and the two matching VBGs work under opposite Braggconditions (e.g., +1 order diffraction versus −1 order diffraction) dueto the opposite propagation directions of the display light at the twomatching VBGs. For example, as shown in FIG. 21, the VBG in thirdgrating 2160 may change the propagation direction of the display lightfrom a downward direction to a rightward direction, while the VBG infirst grating 2130 may change the propagation direction of the displaylight from a rightward direction to a downward direction. Thus, thedispersion caused by first grating 2130 may be opposite to thedispersion caused by third grating 2160 to reduce or minimize theoverall dispersion.

Because first grating 2130 and second grating 2140 may only have a smallnumber (e.g., no greater than 50) of VBGs and exposures, first grating2130 may also be placed in the see-through region to overlap with secondgrating 2140, thus reducing the size of the waveguide display. The totalnumber of VBGs and exposures in a given see-through region may be lessthan, for example, 100 or fewer (e.g., no more than about 40 in firstgrating 2130 and no more than 50 in second grating 2140). Thus, thedisplay haze may be reduced significantly compared with the case where500 or more VBGs are recorded in the see-through region.

In some embodiments, because of the fewer exposures (e.g., smallernumber of gratings in a multiplexed grating), the multiplexed gratingmay not be able to cover the full visible light spectrum and/or the fullFOV, and thus some light information (in some spectral or FOV ranges)may be lost. According to certain embodiments, in order to improve thepower efficiency and to cover a broader spectrum, additional gratingsmay be added at different spatial locations, such as different x, y, orz locations, to spatially multiplex the gratings. In this way, light ina broader bandwidth may be diffracted at a higher diffraction efficiencyby the combination of the gratings to the eyebox. This may also help toincrease the pupil replication density and make the light more uniformin the eyebox.

FIG. 22A illustrates another example of a volume Bragg grating-basedwaveguide display 2200 with exit pupil expansion, dispersion reduction,form-factor reduction, and power efficiency improvement according tocertain embodiments. As waveguide display 2100, waveguide display 2200may include a substrate 2210, which may be similar to substrate 2110.Substrate 2210 may include a first surface 2212 and a second surface2214. Display light from a light source (e.g., LEDs) may be coupled intosubstrate 2210 by an input coupler 2220, and may be reflected by firstsurface 2212 and second surface 2214 through total internal reflection,such that the display light may propagate within substrate 2210. Asdescribed above, input coupler 2220 may include a diffractive coupler,such as a VBG, which may couple display light of different colors intosubstrate 2210 at different diffraction angles.

As waveguide display 2100, waveguide display 2200 may include a firstgrating 2230 and a second grating 2240 formed on first surface 2212and/or second surface 2214. Waveguide display 2200 may also include athird grating 2260 and a fourth grating 2270 formed on first surface2212 and/or second surface 2214. Third grating 2260 and fourth grating2270 may each be a multiplexed VBG that includes multiple VBGs. In someembodiments, third grating 2260, fourth grating 2270, and first grating2230 may be on a same surface of substrate 2210. In some embodiments,third grating 2260, fourth grating 2270, and first grating 2230 may bein different regions of a same grating or a same grating material layer.

In some embodiments, first grating 2230, third grating 2260, and fourthgrating 2270 may each include multiple VBGs. Third grating 2260 andfirst grating 2230 may be recorded in multiple exposures and undersimilar recording conditions (but may be recorded for different exposuredurations to achieve different diffraction efficiencies), such that eachVBG in third grating 2260 may match a respective VBG in first grating2230 (e.g., having the same grating vector in the x-y plane and havingthe same and/or opposite grating vectors in the z direction). Forexample, in some embodiments, a VBG in third grating 2260 and acorresponding VBG in first grating 2230 may have the same grating periodand the same grating slant angle (and thus the same grating vector), andthe same thickness. Fourth grating 2270 and first grating 2230 may alsobe recorded in multiple exposures and under similar recording conditions(but for different exposure durations), such that each VBG in fourthgrating 2270 may match a respective VBG in first grating 2230 (e.g.,having the same grating vector in the x-y plane and having the sameand/or opposite grating vectors in the z direction). In someembodiments, the recording conditions for recording third grating 2260may be different from the recording conditions for recording fourthgrating 2270, such that third grating 2260 and fourth grating 2270 mayhave different Bragg conditions (and different grating vectors) and thusmay diffract light from different FOV ranges and/or wavelength ranges toimprove the overall diffraction efficiency for visible light in a largeFOV range. In some embodiments, third grating 2260 and fourth grating2270 may have similar grating vectors and thus may diffract light fromthe same FOV ranges and/or wavelength ranges with similar or differentdiffraction efficiencies to improve the overall diffraction efficiencyfor light in certain FOV ranges and/or wavelength ranges.

In some embodiments, M VBGs in first grating 2230 that match the M VBGsin third grating 2260 may be recorded in one area (e.g., an upperregion) of first grating 2230, while the other M VBGs in first grating2230 that match the M VBGs in fourth grating 2270 may be recorded in adifferent area (e.g., a lower region) of first grating 2230. In oneexample, third grating 2260 and fourth grating 2270 may each have athickness about 20 μm and may each include about 20 VBGs recordedthrough about 20 exposures. In the example, first grating 2230 may havea thickness about 20 μm and may include about 40 VBGs recorded atdifferent regions through about 40 exposures. Second grating 2240 mayhave a thickness about 20 μm or higher, and may include about 50 VBGsrecorded through about 50 exposures.

Input coupler 2220 may couple the display light from the light sourceinto substrate 2210. The display light may reach third grating 2260directly or may be reflected by first surface 2212 and/or second surface2214 to third grating 2260, where the size of the display light beam maybe slightly larger than that at input coupler 2220. Each VBG in thirdgrating 2260 may diffract a portion of the display light within a FOVrange and a wavelength range that approximately satisfies the Braggcondition of the VBG to an upper region of first grating 2230. Asdescribed above, the upper region of first grating 2230 may include VBGsthat match the VBGs in third grating 2260. Therefore, while the displaylight diffracted by a VBG in third grating 2260 propagates withinsubstrate 2210 (e.g., along a direction shown by a line 2232) throughtotal internal reflection, a portion of the display light may bediffracted by the corresponding VBG in first grating 2230 to secondgrating 2240 each time the display light propagating within substrate2210 reaches first grating 2230.

Display light that is not diffracted by third grating 2260 (e.g., due toa less than 100% diffraction efficiency or due to a small FOV rangeand/or wavelength range near the Bragg condition) may continue topropagate within substrate 2210, and may reach fourth grating 2270. EachVBG in fourth grating 2270 may diffract a portion of the display lightwithin a FOV range and a wavelength range that approximately satisfiesthe Bragg condition of the VBG to a lower region of first grating 2230.As described above, the lower region of first grating 2230 may includeVBGs that match the VBGS in fourth grating 2270. Therefore, while thedisplay light diffracted by a VBG in fourth grating 2270 propagateswithin substrate 2210 (e.g., along a direction shown by a line 2234)through total internal reflection, a portion of the display light may bediffracted by the corresponding VBG in first grating 2230 to secondgrating 2240 each time the display light propagating within substrate2210 reaches first grating 2230. Second grating 2240 may expand thedisplay light from first grating 2230 in a different direction (e.g., inapproximately the y direction) by diffracting a portion of the displaylight to an eyebox 2250 (e.g., at a distance about 18 mm from secondgrating 2240 in +z or −z direction) each time the display lightpropagating within substrate 2210 reaches second grating 2240. In thisway, the display light may be expanded in two dimensions to fill eyebox2250.

FIG. 22B illustrates examples of replicated exit pupils at an eyebox2280 (e.g., eyebox 2250) of volume Bragg grating-based waveguide display2200. The exit pupils may include a first set of exit pupils 2282replicated by gratings 2260, 2230, and 2240, and a second set of exitpupils 2284 replicated by gratings 2270, 2230, and 2240. In embodimentswhere gratings 2260 and gratings 2270 have different grating vectors,the first set of exit pupils 2282 and the second set of exit pupils 2284may correspond to different FOV ranges and/or different wavelengthranges. In embodiments where gratings 2260 and gratings 2270 havesimilar grating vectors, the first set of exit pupils 2282 and thesecond set of exit pupils 2284 may correspond to a same FOV range and/orwavelength range. The first set of exit pupils 2282 and the second setof exit pupils 2284 may overlap or partially overlap. Thus, the pupilreplication density may be increased, and the light may be more uniformin the eyebox, due to the diffraction of display light by two spatiallymultiplexed sets of VBGs.

In addition, the dispersion may be reduced in the two dimensions due tothe dual diffraction in each dimension by a pair of matching gratingsthat operate under opposite Bragg conditions as described above.Furthermore, display light in a broader bandwidth may be diffracted at ahigher diffraction efficiency by the gratings to the eyebox because ofthe lower number of exposures (and thus a higher refractive indexmodulation Δn for each VBG). Thus, the power efficiency of the waveguidedisplay may be improved. In some embodiments, first grating 2230 andsecond grating 2240 may at least partially overlap to reduce the formfactor of waveguide display 2200 as described above.

FIG. 23 illustrates another example of a volume Bragg grating-basedwaveguide display 2300 with exit pupil expansion, dispersion reduction,and form-factor reduction according to certain embodiments. As waveguidedisplay 2100, waveguide display 2300 may include a substrate 2310, whichmay be similar to substrate 2110. Substrate 2310 may include a firstsurface 2312 and a second surface 2314. Display light from a lightsource (e.g., LEDs) may be coupled into substrate 2310 by an inputcoupler 2320, and may be reflected by first surface 2312 and secondsurface 2314 through total internal reflection, such that the displaylight may propagate within substrate 2310. As described above, inputcoupler 2320 may include a diffractive coupler, such as a VBG. Waveguidedisplay 2300 may also include a first grating 2330 and a second grating2340 formed on first surface 2312 and/or second surface 2314. In theexample shown in FIG. 23, first grating 2330 and second grating 2340 maybe at different locations in the x direction, and may overlap in atleast a portion of the see-through region of waveguide display 2300.First grating 2330 and second grating 2340 may be used for dual-axispupil expansion to expand the incident display light beam in twodimensions to fill an eyebox 2350 (e.g., at a distance about 18 mm fromsecond grating 2340 in +z or −z direction) with the display light. Forexample, first grating 2330 may expand the display light beam inapproximately the y direction, while second grating 2340 may expand thedisplay light beam in approximately the x direction.

In addition, waveguide display 2300 may include a third grating 2360formed on first surface 2312 and/or second surface 2314. In someembodiments, third grating 2360 and first grating 2330 may be arrangedat different locations in the y direction on a same surface of substrate2310. In some embodiments, third grating 2360 and first grating 2330 maybe in different regions of a same grating or a same grating materiallayer. In some embodiments, third grating 2360 may be spatially separatefrom first grating 2330. In some embodiments, third grating 2360 andfirst grating 2330 may be recorded in a same number of exposures andunder similar recording conditions (but may be recorded for differentexposure durations to achieve different diffraction efficiencies), suchthat each VBG in third grating 2360 may match a respective VBG in firstgrating 2330 (e.g., having the same grating vector in the x-y plane andhaving the same and/or opposite grating vectors in the z direction).

Input coupler 2320 may couple the display light from the light sourceinto substrate 2310. The display light may propagate approximately alongthe x direction within substrate 2310, and may reach third grating 2360directly or may be reflected by first surface 2312 and/or second surface2314 to third grating 2360. Each VBG in third grating 2360 may diffracta portion of the display light within a FOV range and a wavelength rangethat approximately satisfies the Bragg condition of the VBG downward tofirst grating 2330. While the display light diffracted by a VBG in thirdgrating 2360 propagates within substrate 2310 along a direction (e.g.,approximately in the y direction shown by a line 2332) through totalinternal reflection, a portion of the display light may be diffracted bythe corresponding VBG in first grating 2330 to second grating 2340 eachtime the display light propagating within substrate 2310 reaches firstgrating 2330. Second grating 2340 may then expand the display light fromfirst grating 2330 in a different direction (e.g., approximately in thex direction) by diffracting a portion of the display light to eyebox2350 each time the display light propagating within substrate 2310reaches second grating 2340. Input coupler 2320 and second grating 2340may include matching VBGs (e.g., VBGs with same grating vectors in thex-y plane and the same or opposite grating vectors in the z direction)to reduce the overall dispersion caused by input coupler 2320 and secondgrating 2340. Similarly, gratings 2330 and 2360 may include matchingVBGs (e.g., VBGs with same grating vectors in the x-y plane and havingthe same and/or opposite grating vectors in the z direction) to reducethe overall dispersion caused by gratings 2330 and 2360. Thus, theoverall dispersion by the gratings in waveguide display 2300 may bereduced or minimized.

Each of first grating 2330 and second grating 2340 may have a thicknessless than, for example, 100 μm (e.g., 20 μm), and may include, forexample, fewer than 50 VBGs. Thus, any area in the optical see-throughregion of waveguide display 2300 may include fewer than 100 VBGs. Assuch, the display haze may not be significant. In addition, firstgrating 2330 and second grating 2340 may at least partially overlap toreduce the form factor of waveguide display 2300, and thus the physicaldimensions of waveguide display 2300 may be similar to the physicaldimensions of a lens in a regular pair of eye glasses.

FIG. 23B illustrates an example of a volume Bragg grating-basedwaveguide display 2305 with exit pupil expansion, dispersion reduction,form-factor reduction, and power efficiency improvement according tocertain embodiments. As waveguide display 2300, waveguide display 2305may include a first grating 2335, a second grating 2345, a third grating2365, and a fourth grating 2375 formed on a first surface 2316 and/or asecond surface 2318 of a substrate 2315. First grating 2335, a secondgrating 2345, third grating 2365, and fourth grating 2375 may eachinclude a multiplexed VBG that includes multiple VBGs. In someembodiments, third grating 2365, fourth grating 2375, and first grating2335 may be on a same surface of substrate 2315. In some embodiments,third grating 2365, fourth grating 2375, and first grating 2335 may bein different regions of a same grating or a same grating material layer.

Each VBG in third grating 2365 may have a grating vector matching agrating vector of a respective VBG in first grating 2335 (e.g., havingthe same grating vector in the x-y plane and having the same and/oropposite grating vectors in the z direction), and each VBG in fourthgrating 2375 may have a grating vector matching a grating vector of arespective VBG in fourth grating 2335 (e.g., having the same gratingvector in the x-y plane and having the same and/or opposite gratingvectors in the z direction). In some embodiments, third grating 2365 andfourth grating 2375 may have different grating vectors and thus maydiffract light from different FOV ranges and/or wavelength ranges toimprove the overall diffraction efficiency for visible light in a largeFOV range. In some embodiments, third grating 2365 and fourth grating2375 may have similar grating vectors and thus may diffract light fromthe same FOV ranges and/or wavelength ranges with similar or differentdiffraction efficiencies to improve the overall diffraction efficiencyfor light in certain FOV ranges and/or wavelength ranges.

Input coupler 2325 may couple the display light from the light sourceinto substrate 2315. The display light may reach third grating 2365directly or may be reflected by first surface 2316 and/or second surface2318 to third grating 2365. Each VBG in third grating 2365 may diffracta portion of the display light within a FOV range and a wavelength rangethat approximately satisfies the Bragg condition of the VBG to a leftregion of first grating 2335. While the display light diffracted by aVBG in third grating 2365 propagates within substrate 2315 (e.g., alonga direction shown by a line 2336) through total internal reflection, aportion of the display light may be diffracted by the corresponding VBGin first grating 2335 to second grating 2345 each time the display lightpropagating within substrate 2315 reaches first grating 2335.

Display light that is not diffracted by third grating 2365 (e.g., due toa less than 100% diffraction efficiency or due to a small FOV rangeand/or wavelength range near the Bragg condition) may continue topropagate within substrate 2315, and may reach fourth grating 2375. EachVBG in fourth grating 2375 may diffract a portion of the display lightwithin a FOV range and a wavelength range that approximately satisfiesthe Bragg condition of the VBG to a right region of first grating 2335.While the display light diffracted by a VBG in fourth grating 2375propagates within substrate 2315 (e.g., along a direction shown by aline 2338) through total internal reflection, a portion of the displaylight may be diffracted by the corresponding VBG in first grating 2335to second grating 2345 each time the display light propagating withinsubstrate 2315 reaches first grating 2335.

Second grating 2345 may expand the display light from first grating 2335in a different direction (e.g., in approximately the y direction) bydiffracting a portion of the display light to an eyebox 2355 (e.g., at adistance about 18 mm from second grating 2345 in +z or −z direction)each time the display light propagating within substrate 2315 reachessecond grating 2345. In this way, the display light may be expanded intwo dimensions to fill eyebox 2355. The resultant exit pupils mayinclude a first set of exit pupils replicated by gratings 2365, 2335,and 2345, and a second set of exit pupils replicated by gratings 2375,2335, and 2345. In embodiments where gratings 2365 and gratings 2375have different grating vectors, the first set of exit pupils and thesecond set of exit pupils may correspond to different FOV ranges and/ordifferent wavelength ranges. In embodiments where gratings 2365 andgratings 2375 have similar grating vectors, the first set of exit pupilsand the second set of exit pupils may correspond to a same FOV rangeand/or wavelength range. The first set of exit pupils and the second setof exit pupils may overlap or partially overlap. Thus, the pupilreplication density may be increased, and the light may be more uniformin the eyebox, due to the diffraction of display light by two spatiallymultiplexed sets of VBGs.

FIG. 24A is a front view of an example of a volume Bragg grating-basedwaveguide display 2400 including an image projector 2420 and multiplepolymer layers according to certain embodiments. FIG. 24B is a side viewof the example of volume Bragg grating-based waveguide display 2400including image projector 2420 according to certain embodiments.Waveguide display 2400 may be similar to waveguide display 1600, but mayinclude multiple polymer layers on one or more waveguide plates, wherethe input grating (e.g., input grating 1622), top grating (e.g., firstgrating 1630), and bottom grating (e.g., second grating 1640) may eachbe split into multiple gratings recorded in the multiple polymer layers,where the gratings on each polymer layer may cover different respectiveFOVs and light spectra, and the combination of the multiple polymerlayers may provide the full FOV and spectral coverage. In this way, eachpolymer layer can be thin (e.g., about 20 μm to about 100 μm) and can beexposed for fewer times (e.g., less than about 100) to record fewergratings to reduce haziness, and the overall efficiency of the multiplepolymer layers can still be high for the full FOV and spectrum.

In the example shown in FIGS. 24A and 24B, waveguide display 2400 mayinclude a first polymer layer 2412 and a second polymer layer 2414 onone or more plates or substrates. Each polymer layer 2412 or 2414 mayinclude part of an input grating 2422, a top grating 2430, and a bottomgrating 2440. Display light may be collimated and projected onto inputgrating 2422 by image projector 2420. Input grating 2422 may couple thedisplay light into a waveguide 2410 by diffraction as described abovewith respect to, for example, FIGS. 5 and 6. The display light may reacha first portion 2432 of top grating 2430 and may be diffracted by thefirst portion 2432 of top grating 2430 to change the propagationdirection and reach other portions of top grating 2430, which may eachdiffract the display light towards bottom grating 2440. Bottom grating2440 may then diffract the display light out of waveguide 2410 atdifferent locations to form multiple exit pupils as described above.

First portion 2432 and each of other portions of top grating 2430 mayhave matching grating vectors (e.g., having the same grating vector inthe x-y plane and having the same and/or opposite grating vectors in thez direction, but recorded in different exposure durations to achievedifferent diffraction efficiencies). Therefore, they may compensate forthe dispersion of display light caused by each other to reduce theoverall dispersion, due to the opposite Bragg conditions (e.g., +1 orderand −1 order diffractions) for the diffractions at first portion 2432and each of other portions of top grating 2430. In addition, inputgrating 2422 and bottom grating 2440 may have matching grating vectors(e.g., having the same grating vector in the x-y plane and having thesame or opposite grating vectors in the z direction, but recorded indifferent exposure durations to achieve different diffractionefficiencies), where input grating 2422 may couple the display lightinto waveguide 2410, while bottom grating 2440 may couple the displaylight out of waveguide 2410. Therefore, input grating 2422 and bottomgrating 2440 may compensate for the dispersion of display light causedby each other to reduce the overall dispersion, due to the oppositediffraction directions and opposite Bragg conditions (e.g., +1 order and−1 order diffractions) for the diffractions at input grating 2422 andbottom grating 2440. In this way, the dispersion by first portion 2432and each of other portions of top grating 2430 may be canceled out, andthe dispersion by input grating 2422 and bottom grating 2440 may also becanceled out. Therefore, the overall dispersion of the display light bywaveguide display 2400 can be minimized in any direction. As such, ahigher resolution of the displayed image may be achieved even if thepolymer layers 2412 and 2414 are thin and transmissive VBGs are recordedin the thin polymer layers.

FIG. 25 illustrates an example of a volume Bragg grating-based waveguidedisplay 2500 including multiple grating layers for different fields ofview and/or light wavelengths according to certain embodiments. Inwaveguide display 2500, gratings may be spatially multiplexed along thez direction. For example, waveguide display 2500 may include multiplesubstrates, such as substrates 2510, 2512, 2514, and the like. Thesubstrates may include a same material or materials having similarrefractive indexes. One or more VBGs (e.g., VBGs 2520, 2522, 2524, etc.)may be made on each substrate, such as recorded in a holographicmaterial layer formed on the substrate. The VBGs may be reflectiongratings or transmission gratings. The substrates with the VBGs may bearranged in a substrate stack along the z direction for spatialmultiplexing. Each VBG may be a multiplexed VBG that includes multiplegratings designed for different Bragg conditions to couple display lightin different wavelength ranges and/or different FOVs into or out of thewaveguide.

In the example shown in FIG. 25, VBG 2520 may couple light 2534 from thepositive field of view into the waveguide as shown by light 2544 withinthe waveguide. VBG 2522 may couple light 2530 from around 0° field ofview into the waveguide as shown by light 2540 within the waveguide. VBG2524 may couple light 2532 from the negative field of view into thewaveguide as shown by light 2542 within the waveguide. As describedabove, each of VBGs 2520, 2522, and 2524 may be a multiplexed VBG withmany exposures, and thus may couple light from different FOV ranges intoor out of the waveguide.

In some embodiments, because the diffraction efficiency of atransmission grating may be polarization sensitive and the incomingdisplay light may be unpolarized, some components of the display lightmay not be diffracted by the grating and thus the efficiency of thewaveguide display may be reduced. To improve the efficiency forunpolarized light or light in a certain polarization state, apolarization convertor and two spatially multiplexed gratings may beused to couple the display light into or out of the waveguide.

FIG. 26 illustrates an example of a waveguide display 2600 including twomultiplexed volume Bragg gratings 2610 and 2640 and a polarizationconvertor 2630 between the two multiplexed volume Bragg gratings 2610and 2640 according to certain embodiments. A first VBG 2610 may beformed on a substrate 2620 or on a surface of polarization convertor2630. A second VBG 2640 may be formed on a substrate 2650 or on anothersurface of polarization convertor 2630.

Unpolarized light 2602 may include s-polarized light and p-polarizedlight. First VBG 2610 may diffract a majority of the s-polarized lightand a portion of the p-polarized light as shown by diffracted light2604. Diffracted light 2604 may be partially converted by polarizationconvertor 2630 and pass through second VBG 2640 without being diffractedby second VBG 2640 as shown by transmitted light 2606 because the Braggcondition is not satisfied. The portion 2608 of the p-polarized lightthat is not diffracted by first VBG 2610 may pass through polarizationconvertor 2630 and may be converted into s-polarized light and may bediffracted by second VBG 2640, where the diffracted light 2612 may havethe same propagation direction as transmitted light 2606. In this way,unpolarized light 2602 may be more efficiently diffracted by waveguidedisplay 2600.

External light (e.g., from an external light source, such as a lamp orthe sun) may be reflected at a surface of a grating coupler and back tothe grating coupler, where the reflected light may be diffracted by thegrating coupler to generate rainbow images. In some waveguide display,ambient light with a large incident angle outside of the see-throughfield of view of the waveguide display may also be diffracted by thegrating couplers to generate rainbow images. According to someembodiments, additional structures, such as a reflective coating layer(e.g., for light from a large see-through FOV) and/or an antireflectivecoating layer (e.g., for light from a small see-through FOV), may beused in the waveguide display to reduce optical artifacts, such asrainbow effects. For example, an angular-selective transmissive layermay be placed in front of (or behind) the waveguide and the gratingcoupler of a waveguide display to reduce the artifacts caused byexternal light source. The angular-selective transmissive layer may beconfigured to reflect, diffract, or absorb ambient light with anincident angle greater than one half of the see-through field of view ofthe waveguide display, while allowing ambient light within thesee-through field of view of the near-eye display to pass through andreach user's eyes with little or no loss. The angular-selectivetransmissive layer may include, for example, coating that may includeone or more dielectric layers, diffractive elements such as gratings(e.g., meta-gratings), nanostructures (e.g., nanowires, nano-pillars,nano-prisms, nano-pyramids), and the like.

FIG. 27 illustrates an example of a waveguide display 2700 including ananti-reflection layer 2750 and an angular-selective transmissive layer2740 according to certain embodiments. Waveguide display 2700 mayinclude a waveguide 2710 and a grating coupler 2720 at the bottomsurface of waveguide 2710. Grating coupler 2720 may be similar to thegrating couplers described above. External light 2730 incident onwaveguide 2710 may be refracted into waveguide 2710 as external light2732 and may then be diffracted by grating coupler 2720. The diffractedlight may include a 0^(th) order diffraction 2734 (e.g., refractivediffraction) and a −1st order diffraction (not shown). The height,period, and/or slant angle of grating coupler 2720 may be configuredsuch that the −1st order diffraction may be reduced or minimized for theexternal light.

Waveguide display 2700 may include anti-reflection layer 2750 on bottomsurface 2722 of grating coupler 2720. Anti-reflection layer 2750 mayinclude, for example, one or more dielectric thin film layers or otheranti-reflection layers coated on bottom surface 2722, and may be used toreduce the reflection of the external light at bottom surface 2722.Thus, little or no external light may be reflected at bottom surface2722 of grating coupler 2720 back to grating coupler 2720, and thereforethe rainbow ghost that might otherwise be formed due to the diffractionof external light reflected at bottom surface 2722 by grating coupler2720 may be reduced or minimized. Some portions of the display light maybe diffracted by grating coupler 2720 and may be coupled out ofwaveguide 2710 towards user's eyes (e.g., due to −1^(st) orderdiffraction). Anti-reflection layer 2750 may also help to reduce thereflection of the portions of the display light that are coupled out ofwaveguide 2710 by grating coupler 2720.

Angular-selective transmissive layer 2740 may be coated on the topsurface of waveguide 2710 or grating coupler 2720. Angular-selectivetransmissive layer 2740 may have a high reflectivity, high diffractionefficiency, or high absorption for incident light with an incident anglegreater than a certain threshold value, and may have a low loss forincident light with an incident angle lower than the threshold value.The threshold value may be determined based on the see-through field ofview of waveguide display 2700. For example, incident light 2760 with anincident angle greater than the see-through field of view may be mostlyreflected, diffracted, or absorbed by angular-selective transmissivelayer 2740, and thus may not reach waveguide 2710. External light 2730with an incident angle within the see-through field of view may mostlypass through angular-selective transmissive layer and waveguide 2710,and may be refracted or diffracted by grating coupler 2720.

The angular-selective transmissive layer 2740 described above may beimplemented in various ways. In some embodiments, the angular-selectivetransmissive layer may include one or more dielectric layers (or airgap). Each dielectric layer may have a respective refractive index, andadjacent dielectric layers may have different refractive indexes. Insome embodiments, the angular-selective transmissive layer may include,for example, micro mirrors or prisms, grating, meta-gratings, nanowires,nano-pillars, or other micro- or nano-structures. In some examples, theangular-selective transmissive layer may include gratings (e.g.,surface-relief gratings or holographic gratings) with small gratingperiods formed on a substrate. The gratings may only diffract light withlarge incidence angles (e.g., about 75° to about 90°) and the diffractedlight may propagate in directions such that the diffracted light may notreach the eyebox. The grating period may be, for example, less than 300nm (e.g., about 200 nm) such that the angular-selective transmissivelayer may not affect light within the see-through field of view. In someexamples, the angular-selective transmissive layer may includemicro-scale or nano-scale anisotropic structures that may reflect,diffract, or absorb incident light with large incident angles. Theanisotropic structures may include, for example, large-aspect-rationanoparticles aligned and immersed in transparent media, nanowirearrays, certain liquid crystal materials, and the like.

Embodiments of the invention may be used to implement components of anartificial reality system or may be implemented in conjunction with anartificial reality system. Artificial reality is a form of reality thathas been adjusted in some manner before presentation to a user, whichmay include, for example, a virtual reality (VR), an augmented reality(AR), a mixed reality (MR), a hybrid reality, or some combination and/orderivatives thereof. Artificial reality content may include completelygenerated content or generated content combined with captured (e.g.,real-world) content. The artificial reality content may include video,audio, haptic feedback, or some combination thereof, and any of whichmay be presented in a single channel or in multiple channels (such asstereo video that produces a three-dimensional effect to the viewer).Additionally, in some embodiments, artificial reality may also beassociated with applications, products, accessories, services, or somecombination thereof, that are used to, for example, create content in anartificial reality and/or are otherwise used in (e.g., performactivities in) an artificial reality. The artificial reality system thatprovides the artificial reality content may be implemented on variousplatforms, including a head-mounted display (HMD) connected to a hostcomputer system, a standalone HMD, a mobile device or computing system,or any other hardware platform capable of providing artificial realitycontent to one or more viewers.

FIG. 28 is a simplified block diagram of an example electronic system2800 of an example near-eye display (e.g., HMD device) for implementingsome of the examples disclosed herein. Electronic system 2800 may beused as the electronic system of an HMD device or other near-eyedisplays described above. In this example, electronic system 2800 mayinclude one or more processor(s) 2810 and a memory 2820. Processor(s)2810 may be configured to execute instructions for performing operationsat a number of components, and can be, for example, a general-purposeprocessor or microprocessor suitable for implementation within aportable electronic device. Processor(s) 2810 may be communicativelycoupled with a plurality of components within electronic system 2800. Torealize this communicative coupling, processor(s) 2810 may communicatewith the other illustrated components across a bus 2840. Bus 2840 may beany subsystem adapted to transfer data within electronic system 2800.Bus 2840 may include a plurality of computer buses and additionalcircuitry to transfer data.

Memory 2820 may be coupled to processor(s) 2810. In some embodiments,memory 2820 may offer both short-term and long-term storage and may bedivided into several units. Memory 2820 may be volatile, such as staticrandom access memory (SRAM) and/or dynamic random access memory (DRAM)and/or non-volatile, such as read-only memory (ROM), flash memory, andthe like. Furthermore, memory 2820 may include removable storagedevices, such as secure digital (SD) cards. Memory 2820 may providestorage of computer-readable instructions, data structures, programmodules, and other data for electronic system 2800. In some embodiments,memory 2820 may be distributed into different hardware modules. A set ofinstructions and/or code might be stored on memory 2820. Theinstructions might take the form of executable code that may beexecutable by electronic system 2800, and/or might take the form ofsource and/or installable code, which, upon compilation and/orinstallation on electronic system 2800 (e.g., using any of a variety ofgenerally available compilers, installation programs,compression/decompression utilities, etc.), may take the form ofexecutable code.

In some embodiments, memory 2820 may store a plurality of applicationmodules 2822 through 2824, which may include any number of applications.Examples of applications may include gaming applications, conferencingapplications, video playback applications, or other suitableapplications. The applications may include a depth sensing function oreye tracking function. Application modules 2822-4924 may includeparticular instructions to be executed by processor(s) 2810. In someembodiments, certain applications or parts of application modules2822-4924 may be executable by other hardware modules 2880. In certainembodiments, memory 2820 may additionally include secure memory, whichmay include additional security controls to prevent copying or otherunauthorized access to secure information.

In some embodiments, memory 2820 may include an operating system 2825loaded therein. Operating system 2825 may be operable to initiate theexecution of the instructions provided by application modules 2822-4924and/or manage other hardware modules 2880 as well as interfaces with awireless communication subsystem 2830 which may include one or morewireless transceivers. Operating system 2825 may be adapted to performother operations across the components of electronic system 2800including threading, resource management, data storage control and othersimilar functionality.

Wireless communication subsystem 2830 may include, for example, aninfrared communication device, a wireless communication device and/orchipset (such as a Bluetooth® device, an IEEE 802.11 device, a Wi-Fidevice, a WiMax device, cellular communication facilities, etc.), and/orsimilar communication interfaces. Electronic system 2800 may include oneor more antennas 2834 for wireless communication as part of wirelesscommunication subsystem 2830 or as a separate component coupled to anyportion of the system. Depending on desired functionality, wirelesscommunication subsystem 2830 may include separate transceivers tocommunicate with base transceiver stations and other wireless devicesand access points, which may include communicating with different datanetworks and/or network types, such as wireless wide-area networks(WWANs), wireless local area networks (WLANs), or wireless personal areanetworks (WPANs). A WWAN may be, for example, a WiMax (IEEE 802.16)network. A WLAN may be, for example, an IEEE 802.11x network. A WPAN maybe, for example, a Bluetooth network, an IEEE 802.15x, or some othertypes of network. The techniques described herein may also be used forany combination of WWAN, WLAN, and/or WPAN. Wireless communicationssubsystem 2830 may permit data to be exchanged with a network, othercomputer systems, and/or any other devices described herein. Wirelesscommunication subsystem 2830 may include a means for transmitting orreceiving data, such as identifiers of HMD devices, position data, ageographic map, a heat map, photos, or videos, using antenna(s) 2834 andwireless link(s) 2832. Wireless communication subsystem 2830,processor(s) 2810, and memory 2820 may together comprise at least a partof one or more of a means for performing some functions disclosedherein.

Embodiments of electronic system 2800 may also include one or moresensors 2890. Sensor(s) 2890 may include, for example, an image sensor,an accelerometer, a pressure sensor, a temperature sensor, a proximitysensor, a magnetometer, a gyroscope, an inertial sensor (e.g., a modulethat combines an accelerometer and a gyroscope), an ambient lightsensor, or any other similar module operable to provide sensory outputand/or receive sensory input, such as a depth sensor or a positionsensor. For example, in some implementations, sensor(s) 2890 may includeone or more inertial measurement units (IMUs) and/or one or moreposition sensors. An IMU may generate calibration data indicating anestimated position of the HMD device relative to an initial position ofthe HMD device, based on measurement signals received from one or moreof the position sensors. A position sensor may generate one or moremeasurement signals in response to motion of the HMD device. Examples ofthe position sensors may include, but are not limited to, one or moreaccelerometers, one or more gyroscopes, one or more magnetometers,another suitable type of sensor that detects motion, a type of sensorused for error correction of the IMU, or some combination thereof. Theposition sensors may be located external to the IMU, internal to theIMU, or some combination thereof. At least some sensors may use astructured light pattern for sensing.

Electronic system 2800 may include a display module 2860. Display module2860 may be a near-eye display, and may graphically present information,such as images, videos, and various instructions, from electronic system2800 to a user. Such information may be derived from one or moreapplication modules 2822-4924, virtual reality engine 2826, one or moreother hardware modules 2880, a combination thereof, or any othersuitable means for resolving graphical content for the user (e.g., byoperating system 2825). Display module 2860 may use liquid crystaldisplay (LCD) technology, light-emitting diode (LED) technology(including, for example, OLED, ILED, μLED, AMOLED, TOLED, etc.), lightemitting polymer display (LPD) technology, or some other displaytechnology.

Electronic system 2800 may include a user input/output module 2870. Userinput/output module 2870 may allow a user to send action requests toelectronic system 2800. An action request may be a request to perform aparticular action. For example, an action request may be to start or endan application or to perform a particular action within the application.User input/output module 2870 may include one or more input devices.Example input devices may include a touchscreen, a touch pad,microphone(s), button(s), dial(s), switch(es), a keyboard, a mouse, agame controller, or any other suitable device for receiving actionrequests and communicating the received action requests to electronicsystem 2800. In some embodiments, user input/output module 2870 mayprovide haptic feedback to the user in accordance with instructionsreceived from electronic system 2800. For example, the haptic feedbackmay be provided when an action request is received or has beenperformed.

Electronic system 2800 may include a camera 2850 that may be used totake photos or videos of a user, for example, for tracking the user'seye position. Camera 2850 may also be used to take photos or videos ofthe environment, for example, for VR, AR, or MR applications. Camera2850 may include, for example, a complementary metal-oxide-semiconductor(CMOS) image sensor with a few millions or tens of millions of pixels.In some implementations, camera 2850 may include two or more camerasthat may be used to capture 3-D images.

In some embodiments, electronic system 2800 may include a plurality ofother hardware modules 2880. Each of other hardware modules 2880 may bea physical module within electronic system 2800. While each of otherhardware modules 2880 may be permanently configured as a structure, someof other hardware modules 2880 may be temporarily configured to performspecific functions or temporarily activated. Examples of other hardwaremodules 2880 may include, for example, an audio output and/or inputmodule (e.g., a microphone or speaker), a near field communication (NFC)module, a rechargeable battery, a battery management system, awired/wireless battery charging system, etc. In some embodiments, one ormore functions of other hardware modules 2880 may be implemented insoftware.

In some embodiments, memory 2820 of electronic system 2800 may alsostore a virtual reality engine 2826. Virtual reality engine 2826 mayexecute applications within electronic system 2800 and receive positioninformation, acceleration information, velocity information, predictedfuture positions, or some combination thereof of the HMD device from thevarious sensors. In some embodiments, the information received byvirtual reality engine 2826 may be used for producing a signal (e.g.,display instructions) to display module 2860. For example, if thereceived information indicates that the user has looked to the left,virtual reality engine 2826 may generate content for the HMD device thatmirrors the user's movement in a virtual environment. Additionally,virtual reality engine 2826 may perform an action within an applicationin response to an action request received from user input/output module2870 and provide feedback to the user. The provided feedback may bevisual, audible, or haptic feedback. In some implementations,processor(s) 2810 may include one or more GPUs that may execute virtualreality engine 2826.

In various implementations, the above-described hardware and modules maybe implemented on a single device or on multiple devices that cancommunicate with one another using wired or wireless connections. Forexample, in some implementations, some components or modules, such asGPUs, virtual reality engine 2826, and applications (e.g., trackingapplication), may be implemented on a console separate from thehead-mounted display device. In some implementations, one console may beconnected to or support more than one HMD.

In alternative configurations, different and/or additional componentsmay be included in electronic system 2800. Similarly, functionality ofone or more of the components can be distributed among the components ina manner different from the manner described above. For example, in someembodiments, electronic system 2800 may be modified to include othersystem environments, such as an AR system environment and/or an MRenvironment.

The methods, systems, and devices discussed above are examples. Variousembodiments may omit, substitute, or add various procedures orcomponents as appropriate. For instance, in alternative configurations,the methods described may be performed in an order different from thatdescribed, and/or various stages may be added, omitted, and/or combined.Also, features described with respect to certain embodiments may becombined in various other embodiments. Different aspects and elements ofthe embodiments may be combined in a similar manner. Also, technologyevolves and, thus, many of the elements are examples that do not limitthe scope of the disclosure to those specific examples.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, systems, structures, and techniques have been shown withoutunnecessary detail in order to avoid obscuring the embodiments. Thisdescription provides example embodiments only, and is not intended tolimit the scope, applicability, or configuration of the invention.Rather, the preceding description of the embodiments will provide thoseskilled in the art with an enabling description for implementing variousembodiments. Various changes may be made in the function and arrangementof elements without departing from the spirit and scope of the presentdisclosure.

Also, some embodiments were described as processes depicted as flowdiagrams or block diagrams. Although each may describe the operations asa sequential process, many of the operations may be performed inparallel or concurrently. In addition, the order of the operations maybe rearranged. A process may have additional steps not included in thefigure. Furthermore, embodiments of the methods may be implemented byhardware, software, firmware, middleware, microcode, hardwaredescription languages, or any combination thereof. When implemented insoftware, firmware, middleware, or microcode, the program code or codesegments to perform the associated tasks may be stored in acomputer-readable medium such as a storage medium. Processors mayperform the associated tasks.

It will be apparent to those skilled in the art that substantialvariations may be made in accordance with specific requirements. Forexample, customized or special-purpose hardware might also be used,and/or particular elements might be implemented in hardware, software(including portable software, such as applets, etc.), or both. Further,connection to other computing devices such as network input/outputdevices may be employed.

With reference to the appended figures, components that can includememory can include non-transitory machine-readable media. The term“machine-readable medium” and “computer-readable medium” may refer toany storage medium that participates in providing data that causes amachine to operate in a specific fashion. In embodiments providedhereinabove, various machine-readable media might be involved inproviding instructions/code to processing units and/or other device(s)for execution. Additionally or alternatively, the machine-readable mediamight be used to store and/or carry such instructions/code. In manyimplementations, a computer-readable medium is a physical and/ortangible storage medium. Such a medium may take many forms, including,but not limited to, non-volatile media, volatile media, and transmissionmedia. Common forms of computer-readable media include, for example,magnetic and/or optical media such as compact disk (CD) or digitalversatile disk (DVD), punch cards, paper tape, any other physical mediumwith patterns of holes, a RAM, a programmable read-only memory (PROM),an erasable programmable read-only memory (EPROM), a FLASH-EPROM, anyother memory chip or cartridge, a carrier wave as described hereinafter,or any other medium from which a computer can read instructions and/orcode. A computer program product may include code and/ormachine-executable instructions that may represent a procedure, afunction, a subprogram, a program, a routine, an application (App), asubroutine, a module, a software package, a class, or any combination ofinstructions, data structures, or program statements.

Those of skill in the art will appreciate that information and signalsused to communicate the messages described herein may be representedusing any of a variety of different technologies and techniques. Forexample, data, instructions, commands, information, signals, bits,symbols, and chips that may be referenced throughout the abovedescription may be represented by voltages, currents, electromagneticwaves, magnetic fields or particles, optical fields or particles, or anycombination thereof.

Terms, “and” and “or” as used herein, may include a variety of meaningsthat are also expected to depend at least in part upon the context inwhich such terms are used. Typically, “or” if used to associate a list,such as A, B, or C, is intended to mean A, B, and C, here used in theinclusive sense, as well as A, B, or C, here used in the exclusivesense. In addition, the term “one or more” as used herein may be used todescribe any feature, structure, or characteristic in the singular ormay be used to describe some combination of features, structures, orcharacteristics. However, it should be noted that this is merely anillustrative example and claimed subject matter is not limited to thisexample. Furthermore, the term “at least one of” if used to associate alist, such as A, B, or C, can be interpreted to mean any combination ofA, B, and/or C, such as A, AB, AC, BC, AA, ABC, AAB, AABBCCC, etc.

Further, while certain embodiments have been described using aparticular combination of hardware and software, it should be recognizedthat other combinations of hardware and software are also possible.Certain embodiments may be implemented only in hardware, or only insoftware, or using combinations thereof. In one example, software may beimplemented with a computer program product containing computer programcode or instructions executable by one or more processors for performingany or all of the steps, operations, or processes described in thisdisclosure, where the computer program may be stored on a non-transitorycomputer readable medium. The various processes described herein can beimplemented on the same processor or different processors in anycombination.

Where devices, systems, components or modules are described as beingconfigured to perform certain operations or functions, suchconfiguration can be accomplished, for example, by designing electroniccircuits to perform the operation, by programming programmableelectronic circuits (such as microprocessors) to perform the operationsuch as by executing computer instructions or code, or processors orcores programmed to execute code or instructions stored on anon-transitory memory medium, or any combination thereof. Processes cancommunicate using a variety of techniques, including, but not limitedto, conventional techniques for inter-process communications, anddifferent pairs of processes may use different techniques, or the samepair of processes may use different techniques at different times.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that additions, subtractions, deletions, and other modificationsand changes may be made thereunto without departing from the broaderspirit and scope as set forth in the claims. Thus, although specificembodiments have been described, these are not intended to be limiting.Various modifications and equivalents are within the scope of thefollowing claims.

What is claimed is:
 1. A waveguide display comprising: a substratetransparent to visible light; and a first volume Bragg grating (VBG), asecond VBG, and a third VBG coupled to the substrate, wherein the firstVBG is configured to couple display light into the substrate as guidedwave towards a first region of the second VBG; wherein the second VBG isconfigured to: diffract, at the first region of the second VBG, thedisplay light from the first VBG to a first direction; and diffract, attwo or more regions of the second VBG along the first direction, thedisplay light from the first region to a second direction towards thethird VBG; and wherein the third VBG is configured to couple the displaylight from each of the two or more regions of the second VBG out of thesubstrate at two or more regions of the third VBG along the seconddirection.
 2. The waveguide display of claim 1, wherein the first VBGand the third VBG have a same grating vector in a plane perpendicular toa surface normal direction of the substrate.
 3. The waveguide display ofclaim 1, wherein the first VBG, the second VBG, and the third VBG areconfigured to diffract the display light from a same field of view rangeand in a same wavelength range.
 4. The waveguide display of claim 1,wherein each of the first VBG, the second VBG, and the third VBGincludes a reflective VBG or a transmissive VBG.
 5. The waveguidedisplay of claim 1, wherein: the third VBG includes a transmissive VBG;and the second VBG overlaps with the third VBG in a see-through regionof the waveguide display.
 6. The waveguide display of claim 1, whereinat least one of the first VBG, the second VBG, or the third VBG includesa multiplexed VBG.
 7. The waveguide display of claim 6, wherein: thefirst VBG includes a first set of VBGs; the third VBG includes a secondset of VBGs; and each VBG in the first set of VBGs and a correspondingVBG in the second set of VBGs have a same grating vector in a planeperpendicular to a surface normal direction of the substrate and areconfigured to diffract the display light from a same field of view rangeand in a same wavelength range.
 8. The waveguide display of claim 6,wherein at least one of the first VBG, the second VBG, or the third VBGincludes VBGs in two or more holographic material layers.
 9. Thewaveguide display of claim 8, further comprising a polarizationconvertor between two holographic material layers of the two or moreholographic material layers.
 10. The waveguide display of claim 1,further comprising an anti-reflection layer configured to reducereflection of ambient light into the substrate.
 11. The waveguidedisplay of claim 1, further comprising an angular-selective transmissivelayer configured to reflect, diffract, or absorb ambient light incidenton the angular-selective transmissive layer with an incidence anglegreater than a threshold value.
 12. The waveguide display of claim 1,wherein: each of the second VBG and the third VBG is characterized by arespective thickness less than 100 μm; and the waveguide display ischaracterized by an angular resolution less than 2 arcminutes.
 13. Thewaveguide display of claim 1, wherein the first region of the second VBGand a second region of the two or more regions of the second VBG have asame grating vector in a plane perpendicular to a surface normaldirection of the substrate.
 14. The waveguide display of claim 1,further comprising: a light source configured to generate the displaylight; and projector optics configure to collimate the display light anddirect the display light to the first VBG.
 15. A waveguide displaycomprising: a substrate transparent to visible light; a couplerconfigured to couple display light into the substrate as guided wave inthe substrate; and a first volume Bragg grating (VBG) and a second VBGcoupled to the substrate, wherein the first VBG is configured to:diffract, at a first region of the first VBG, the display light in thesubstrate to a first direction; and diffract, at two or more regions ofthe first VBG along the first direction, the display light from thefirst region to a second direction towards the second VBG; and whereinthe second VBG is configured to couple the display light from each ofthe two or more regions of the first VBG out of the substrate at two ormore regions of the second VBG along the second direction.
 16. Thewaveguide display of claim 15, wherein: the first VBG is characterizedby a thickness less than 100 μm; and the waveguide display ischaracterized by an angular resolution less than 2 arcminutes.
 17. Thewaveguide display of claim 15, wherein: the second VBG includes atransmissive VBG; and the first VBG overlaps with the second VBG in asee-through region of the waveguide display.
 18. The waveguide displayof claim 15, wherein at least one of the first VBG or the second VBGincludes VBGs in two or more holographic material layers.
 19. Thewaveguide display of claim 15, wherein the coupler includes adiffractive coupler, a refractive coupler, or a reflective coupler. 20.The waveguide display of claim 15, wherein at least one of the first VBGor the second VBG includes a multiplexed VBG.
 21. The waveguide displayof claim 15, wherein each of the first VBG and the second VBG includes atransmissive VBG or a reflective VBG.
 22. The waveguide display of claim15, wherein the first region of the first VBG and a second region of thetwo or more regions of the first VBG have a same grating vector in aplane perpendicular to a surface normal direction of the substrate.